NushellでGitのブランチ名を補完する

詳しいことはわかってないですが、とりあえずNushellでgit checkoutするときにブランチ名を補完できるようにする方法がわかったのでメモです。

[過去のコミット](https://github.com/nushell/nushell/blob/7a820b1304b45f1bbf2d2f13e44af4659acbaeb4/docs/sample_config/default_config.nu)から削除されたらしき設定を見つけて、
```
def "nu-complete git branches" [] {
    ^git branch | lines | each { |line| $line | str replace '[\*\+] ' '' | str trim }
  }

  def "nu-complete git remotes" [] {
    ^git remote | lines | each { |line| $line | str trim }
  }

  # Download objects and refs from another repository
  export extern "git fetch" [
    repository?: string@"nu-complete git remotes" # name of the repository to fetch
    branch?: string@"nu-complete git branches" # name of the branch to fetch
    --all                                         # Fetch all remotes
    --append(-a)                                  # Append ref names and object names to .git/FETCH_HEAD
    --atomic                                      # Use an atomic transaction to update local refs.
    --depth: int                                  # Limit fetching to n commits from the tip
    --deepen: int                                 # Limit fetching to n commits from the current shallow boundary
    --shallow-since: string                       # Deepen or shorten the history by date
    --shallow-exclude: string                     # Deepen or shorten the history by branch/tag
    --unshallow                                   # Fetch all available history
    --update-shallow                              # Update .git/shallow to accept new refs
    --negotiation-tip: string                     # Specify which commit/glob to report while fetching
    --negotiate-only                              # Do not fetch, only print common ancestors
    --dry-run                                     # Show what would be done
    --write-fetch-head                            # Write fetched refs in FETCH_HEAD (default)
    --no-write-fetch-head                         # Do not write FETCH_HEAD
    --force(-f)                                   # Always update the local branch
    --keep(-k)                                    # Keep dowloaded pack
    --multiple                                    # Allow several arguments to be specified
    --auto-maintenance                            # Run 'git maintenance run --auto' at the end (default)
    --no-auto-maintenance                         # Don't run 'git maintenance' at the end
    --auto-gc                                     # Run 'git maintenance run --auto' at the end (default)
    --no-auto-gc                                  # Don't run 'git maintenance' at the end
    --write-commit-graph                          # Write a commit-graph after fetching
    --no-write-commit-graph                       # Don't write a commit-graph after fetching
    --prefetch                                    # Place all refs into the refs/prefetch/ namespace
    --prune(-p)                                   # Remove obsolete remote-tracking references
    --prune-tags(-P)                              # Remove any local tags that do not exist on the remote
    --no-tags(-n)                                 # Disable automatic tag following
    --refmap: string                              # Use this refspec to map the refs to remote-tracking branches
    --tags(-t)                                    # Fetch all tags
    --recurse-submodules: string                  # Fetch new commits of populated submodules (yes/on-demand/no)
    --jobs(-j): int                               # Number of parallel children
    --no-recurse-submodules                       # Disable recursive fetching of submodules
    --set-upstream                                # Add upstream (tracking) reference
    --submodule-prefix: string                    # Prepend to paths printed in informative messages
    --upload-pack: string                         # Non-default path for remote command
    --quiet(-q)                                   # Silence internally used git commands
    --verbose(-v)                                 # Be verbose
    --progress                                    # Report progress on stderr
    --server-option(-o): string                   # Pass options for the server to handle
    --show-forced-updates                         # Check if a branch is force-updated
    --no-show-forced-updates                      # Don't check if a branch is force-updated
    -4                                            # Use IPv4 addresses, ignore IPv6 addresses
    -6                                            # Use IPv6 addresses, ignore IPv4 addresses
    --help                                        # Display this help message
  ]

  # Check out git branches and files
  export extern "git checkout" [
    ...targets: string@"nu-complete git branches"   # name of the branch or files to checkout
    --conflict: string                              # conflict style (merge or diff3)
    --detach(-d)                                    # detach HEAD at named commit
    --force(-f)                                     # force checkout (throw away local modifications)
    --guess                                         # second guess 'git checkout <no-such-branch>' (default)
    --ignore-other-worktrees                        # do not check if another worktree is holding the given ref
    --ignore-skip-worktree-bits                     # do not limit pathspecs to sparse entries only
    --merge(-m)                                     # perform a 3-way merge with the new branch
    --orphan: string                                # new unparented branch
    --ours(-2)                                      # checkout our version for unmerged files
    --overlay                                       # use overlay mode (default)
    --overwrite-ignore                              # update ignored files (default)
    --patch(-p)                                     # select hunks interactively
    --pathspec-from-file: string                    # read pathspec from file
    --progress                                      # force progress reporting
    --quiet(-q)                                     # suppress progress reporting
    --recurse-submodules: string                    # control recursive updating of submodules
    --theirs(-3)                                    # checkout their version for unmerged files
    --track(-t)                                     # set upstream info for new branch
    -b: string                                      # create and checkout a new branch
    -B: string                                      # create/reset and checkout a branch
    -l                                              # create reflog for new branch
    --help                                          # Display this help message
  ]

  # Push changes
  export extern "git push" [
    remote?: string@"nu-complete git remotes",      # the name of the remote
    ...refs: string@"nu-complete git branches"      # the branch / refspec
    --all                                           # push all refs
    --atomic                                        # request atomic transaction on remote side
    --delete(-d)                                    # delete refs
    --dry-run(-n)                                   # dry run
    --exec: string                                  # receive pack program
    --follow-tags                                   # push missing but relevant tags
    --force-with-lease: string                      # require old value of ref to be at this value
    --force(-f)                                     # force updates
    --ipv4(-4)                                      # use IPv4 addresses only
    --ipv6(-6)                                      # use IPv6 addresses only
    --mirror                                        # mirror all refs
    --no-verify                                     # bypass pre-push hook
    --porcelain                                     # machine-readable output
    --progress                                      # force progress reporting
    --prune                                         # prune locally removed refs
    --push-option(-o): string                       # option to transmit
    --quiet(-q)                                     # be more quiet
    --receive-pack: string                          # receive pack program
    --recurse-submodules: string                    # control recursive pushing of submodules
    --repo: string                                  # repository
    --set-upstream(-u)                              # set upstream for git pull/status
    --signed: string                                # GPG sign the push
    --tags                                          # push tags (can't be used with --all or --mirror)
    --thin                                          # use thin pack
    --verbose(-v)                                   # be more verbose
    --help                                          # Display this help message
  ]
```
それをconfig.nuの末尾に貼り付けます。

最終的なconfig.nuの中身は
```:config.nu
# Nushell Config File
#
# version = 0.79.1

# For more information on defining custom themes, see
# https://www.nushell.sh/book/coloring_and_theming.html
# And here is the theme collection
# https://github.com/nushell/nu_scripts/tree/main/themes
let dark_theme = {
    # color for nushell primitives
    separator: white
    leading_trailing_space_bg: { attr: n } # no fg, no bg, attr none effectively turns this off
    header: green_bold
    empty: blue
    # Closures can be used to choose colors for specific values.
    # The value (in this case, a bool) is piped into the closure.
    bool: {|| if $in { 'light_cyan' } else { 'light_gray' } }
    int: white
    filesize: {|e|
      if $e == 0b {
        'white'
      } else if $e < 1mb {
        'cyan'
      } else { 'blue' }
    }
    duration: white
    date: {|| (date now) - $in |
      if $in < 1hr {
        'purple'
      } else if $in < 6hr {
        'red'
      } else if $in < 1day {
        'yellow'
      } else if $in < 3day {
        'green'
      } else if $in < 1wk {
        'light_green'
      } else if $in < 6wk {
        'cyan'
      } else if $in < 52wk {
        'blue'
      } else { 'dark_gray' }
    }
    range: white
    float: white
    string: white
    nothing: white
    binary: white
    cellpath: white
    row_index: green_bold
    record: white
    list: white
    block: white
    hints: dark_gray

    shape_and: purple_bold
    shape_binary: purple_bold
    shape_block: blue_bold
    shape_bool: light_cyan
    shape_closure: green_bold
    shape_custom: green
    shape_datetime: cyan_bold
    shape_directory: cyan
    shape_external: cyan
    shape_externalarg: green_bold
    shape_filepath: cyan
    shape_flag: blue_bold
    shape_float: purple_bold
    # shapes are used to change the cli syntax highlighting
    shape_garbage: { fg: white bg: red attr: b}
    shape_globpattern: cyan_bold
    shape_int: purple_bold
    shape_internalcall: cyan_bold
    shape_list: cyan_bold
    shape_literal: blue
    shape_match_pattern: green
    shape_matching_brackets: { attr: u }
    shape_nothing: light_cyan
    shape_operator: yellow
    shape_or: purple_bold
    shape_pipe: purple_bold
    shape_range: yellow_bold
    shape_record: cyan_bold
    shape_redirection: purple_bold
    shape_signature: green_bold
    shape_string: green
    shape_string_interpolation: cyan_bold
    shape_table: blue_bold
    shape_variable: purple
    shape_vardecl: purple
}

let light_theme = {
    # color for nushell primitives
    separator: dark_gray
    leading_trailing_space_bg: { attr: n } # no fg, no bg, attr none effectively turns this off
    header: green_bold
    empty: blue
    # Closures can be used to choose colors for specific values.
    # The value (in this case, a bool) is piped into the closure.
    bool: {|| if $in { 'dark_cyan' } else { 'dark_gray' } }
    int: dark_gray
    filesize: {|e|
      if $e == 0b {
        'dark_gray'
      } else if $e < 1mb {
        'cyan_bold'
      } else { 'blue_bold' }
    }
    duration: dark_gray
  date: {|| (date now) - $in |
    if $in < 1hr {
      'purple'
    } else if $in < 6hr {
      'red'
    } else if $in < 1day {
      'yellow'
    } else if $in < 3day {
      'green'
    } else if $in < 1wk {
      'light_green'
    } else if $in < 6wk {
      'cyan'
    } else if $in < 52wk {
      'blue'
    } else { 'dark_gray' }
  }
    range: dark_gray
    float: dark_gray
    string: dark_gray
    nothing: dark_gray
    binary: dark_gray
    cellpath: dark_gray
    row_index: green_bold
    record: white
    list: white
    block: white
    hints: dark_gray

    shape_and: purple_bold
    shape_binary: purple_bold
    shape_block: blue_bold
    shape_bool: light_cyan
    shape_closure: green_bold
    shape_custom: green
    shape_datetime: cyan_bold
    shape_directory: cyan
    shape_external: cyan
    shape_externalarg: green_bold
    shape_filepath: cyan
    shape_flag: blue_bold
    shape_float: purple_bold
    # shapes are used to change the cli syntax highlighting
    shape_garbage: { fg: white bg: red attr: b}
    shape_globpattern: cyan_bold
    shape_int: purple_bold
    shape_internalcall: cyan_bold
    shape_list: cyan_bold
    shape_literal: blue
    shape_match_pattern: green
    shape_matching_brackets: { attr: u }
    shape_nothing: light_cyan
    shape_operator: yellow
    shape_or: purple_bold
    shape_pipe: purple_bold
    shape_range: yellow_bold
    shape_record: cyan_bold
    shape_redirection: purple_bold
    shape_signature: green_bold
    shape_string: green
    shape_string_interpolation: cyan_bold
    shape_table: blue_bold
    shape_variable: purple
    shape_vardecl: purple
}

# External completer example
# let carapace_completer = {|spans|
#     carapace $spans.0 nushell $spans | from json
# }


# The default config record. This is where much of your global configuration is setup.
let-env config = {
  # true or false to enable or disable the welcome banner at startup
  show_banner: true
  ls: {
    use_ls_colors: true # use the LS_COLORS environment variable to colorize output
    clickable_links: true # enable or disable clickable links. Your terminal has to support links.
  }
  rm: {
    always_trash: false # always act as if -t was given. Can be overridden with -p
  }
  cd: {
    abbreviations: false # allows `cd s/o/f` to expand to `cd some/other/folder`
  }
  table: {
    mode: rounded # basic, compact, compact_double, light, thin, with_love, rounded, reinforced, heavy, none, other
    index_mode: always # "always" show indexes, "never" show indexes, "auto" = show indexes when a table has "index" column
    show_empty: true # show 'empty list' and 'empty record' placeholders for command output
    trim: {
      methodology: wrapping # wrapping or truncating
      wrapping_try_keep_words: true # A strategy used by the 'wrapping' methodology
      truncating_suffix: "..." # A suffix used by the 'truncating' methodology
    }
  }

  explore: {
    help_banner: true
    exit_esc: true

    command_bar_text: '#C4C9C6'
    # command_bar: {fg: '#C4C9C6' bg: '#223311' }

    status_bar_background: {fg: '#1D1F21' bg: '#C4C9C6' }
    # status_bar_text: {fg: '#C4C9C6' bg: '#223311' }

    highlight: {bg: 'yellow' fg: 'black' }

    status: {
      # warn: {bg: 'yellow', fg: 'blue'}
      # error: {bg: 'yellow', fg: 'blue'}
      # info: {bg: 'yellow', fg: 'blue'}
    }

    try: {
      # border_color: 'red'
      # highlighted_color: 'blue'

      # reactive: false
    }

    table: {
      split_line: '#404040'

      cursor: true

      line_index: true
      line_shift: true
      line_head_top: true
      line_head_bottom: true

      show_head: true
      show_index: true

      # selected_cell: {fg: 'white', bg: '#777777'}
      # selected_row: {fg: 'yellow', bg: '#C1C2A3'}
      # selected_column: blue

      # padding_column_right: 2
      # padding_column_left: 2

      # padding_index_left: 2
      # padding_index_right: 1
    }

    config: {
      cursor_color: {bg: 'yellow' fg: 'black' }

      # border_color: white
      # list_color: green
    }
  }

  history: {
    max_size: 10000 # Session has to be reloaded for this to take effect
    sync_on_enter: true # Enable to share history between multiple sessions, else you have to close the session to write history to file
    file_format: "plaintext" # "sqlite" or "plaintext"
    history_isolation: true # true enables history isolation, false disables it. true will allow the history to be isolated to the current session. false will allow the history to be shared across all sessions.
  }
  completions: {
    case_sensitive: false # set to true to enable case-sensitive completions
    quick: true  # set this to false to prevent auto-selecting completions when only one remains
    partial: true  # set this to false to prevent partial filling of the prompt
    algorithm: "prefix"  # prefix or fuzzy
    external: {
      enable: true # set to false to prevent nushell looking into $env.PATH to find more suggestions, `false` recommended for WSL users as this look up my be very slow
      max_results: 100 # setting it lower can improve completion performance at the cost of omitting some options
      completer: null # check 'carapace_completer' above as an example
    }
  }
  filesize: {
    metric: true # true => KB, MB, GB (ISO standard), false => KiB, MiB, GiB (Windows standard)
    format: "auto" # b, kb, kib, mb, mib, gb, gib, tb, tib, pb, pib, eb, eib, zb, zib, auto
  }
  cursor_shape: {
    emacs: line # block, underscore, line, blink_block, blink_underscore, blink_line (line is the default)
    vi_insert: block # block, underscore, line , blink_block, blink_underscore, blink_line (block is the default)
    vi_normal: underscore # block, underscore, line, blink_block, blink_underscore, blink_line (underscore is the default)
  }
  color_config: $dark_theme   # if you want a light theme, replace `$dark_theme` to `$light_theme`
  use_grid_icons: true
  footer_mode: "25" # always, never, number_of_rows, auto
  float_precision: 2 # the precision for displaying floats in tables
  # buffer_editor: "emacs" # command that will be used to edit the current line buffer with ctrl+o, if unset fallback to $env.EDITOR and $env.VISUAL
  use_ansi_coloring: true
  bracketed_paste: true # enable bracketed paste, currently useless on windows
  edit_mode: emacs # emacs, vi
  shell_integration: true # enables terminal markers and a workaround to arrow keys stop working issue
  render_right_prompt_on_last_line: false # true or false to enable or disable right prompt to be rendered on last line of the prompt.

  hooks: {
    pre_prompt: [{||
      null  # replace with source code to run before the prompt is shown
    }]
    pre_execution: [{||
      null  # replace with source code to run before the repl input is run
    }]
    env_change: {
      PWD: [{|before, after|
        null  # replace with source code to run if the PWD environment is different since the last repl input
      }]
    }
    display_output: {||
      if (term size).columns >= 100 { table -e } else { table }
    }
    command_not_found: {||
      null  # replace with source code to return an error message when a command is not found
    }
  }
  menus: [
      # Configuration for default nushell menus
      # Note the lack of source parameter
      {
        name: completion_menu
        only_buffer_difference: false
        marker: "| "
        type: {
            layout: columnar
            columns: 4
            col_width: 20   # Optional value. If missing all the screen width is used to calculate column width
            col_padding: 2
        }
        style: {
            text: green
            selected_text: green_reverse
            description_text: yellow
        }
      }
      {
        name: history_menu
        only_buffer_difference: true
        marker: "? "
        type: {
            layout: list
            page_size: 10
        }
        style: {
            text: green
            selected_text: green_reverse
            description_text: yellow
        }
      }
      {
        name: help_menu
        only_buffer_difference: true
        marker: "? "
        type: {
            layout: description
            columns: 4
            col_width: 20   # Optional value. If missing all the screen width is used to calculate column width
            col_padding: 2
            selection_rows: 4
            description_rows: 10
        }
        style: {
            text: green
            selected_text: green_reverse
            description_text: yellow
        }
      }
      # Example of extra menus created using a nushell source
      # Use the source field to create a list of records that populates
      # the menu
      {
        name: commands_menu
        only_buffer_difference: false
        marker: "# "
        type: {
            layout: columnar
            columns: 4
            col_width: 20
            col_padding: 2
        }
        style: {
            text: green
            selected_text: green_reverse
            description_text: yellow
        }
        source: { |buffer, position|
            $nu.scope.commands
            | where name =~ $buffer
            | each { |it| {value: $it.name description: $it.usage} }
        }
      }
      {
        name: vars_menu
        only_buffer_difference: true
        marker: "# "
        type: {
            layout: list
            page_size: 10
        }
        style: {
            text: green
            selected_text: green_reverse
            description_text: yellow
        }
        source: { |buffer, position|
            $nu.scope.vars
            | where name =~ $buffer
            | sort-by name
            | each { |it| {value: $it.name description: $it.type} }
        }
      }
      {
        name: commands_with_description
        only_buffer_difference: true
        marker: "# "
        type: {
            layout: description
            columns: 4
            col_width: 20
            col_padding: 2
            selection_rows: 4
            description_rows: 10
        }
        style: {
            text: green
            selected_text: green_reverse
            description_text: yellow
        }
        source: { |buffer, position|
            $nu.scope.commands
            | where name =~ $buffer
            | each { |it| {value: $it.name description: $it.usage} }
        }
      }
  ]
  keybindings: [
    {
      name: completion_menu
      modifier: none
      keycode: tab
      mode: [emacs vi_normal vi_insert]
      event: {
        until: [
          { send: menu name: completion_menu }
          { send: menunext }
        ]
      }
    }
    {
      name: completion_previous
      modifier: shift
      keycode: backtab
      mode: [emacs, vi_normal, vi_insert] # Note: You can add the same keybinding to all modes by using a list
      event: { send: menuprevious }
    }
    {
      name: history_menu
      modifier: control
      keycode: char_r
      mode: emacs
      event: { send: menu name: history_menu }
    }
    {
      name: next_page
      modifier: control
      keycode: char_x
      mode: emacs
      event: { send: menupagenext }
    }
    {
      name: undo_or_previous_page
      modifier: control
      keycode: char_z
      mode: emacs
      event: {
        until: [
          { send: menupageprevious }
          { edit: undo }
        ]
       }
    }
    {
      name: yank
      modifier: control
      keycode: char_y
      mode: emacs
      event: {
        until: [
          {edit: pastecutbufferafter}
        ]
      }
    }
    {
      name: unix-line-discard
      modifier: control
      keycode: char_u
      mode: [emacs, vi_normal, vi_insert]
      event: {
        until: [
          {edit: cutfromlinestart}
        ]
      }
    }
    {
      name: kill-line
      modifier: control
      keycode: char_k
      mode: [emacs, vi_normal, vi_insert]
      event: {
        until: [
          {edit: cuttolineend}
        ]
      }
    }
    # Keybindings used to trigger the user defined menus
    {
      name: commands_menu
      modifier: control
      keycode: char_t
      mode: [emacs, vi_normal, vi_insert]
      event: { send: menu name: commands_menu }
    }
    {
      name: vars_menu
      modifier: alt
      keycode: char_o
      mode: [emacs, vi_normal, vi_insert]
      event: { send: menu name: vars_menu }
    }
    {
      name: commands_with_description
      modifier: control
      keycode: char_s
      mode: [emacs, vi_normal, vi_insert]
      event: { send: menu name: commands_with_description }
    }
  ]
}

def "nu-complete git branches" [] {
    ^git branch | lines | each { |line| $line | str replace '[\*\+] ' '' | str trim }
  }

  def "nu-complete git remotes" [] {
    ^git remote | lines | each { |line| $line | str trim }
  }

  # Download objects and refs from another repository
  export extern "git fetch" [
    repository?: string@"nu-complete git remotes" # name of the repository to fetch
    branch?: string@"nu-complete git branches" # name of the branch to fetch
    --all                                         # Fetch all remotes
    --append(-a)                                  # Append ref names and object names to .git/FETCH_HEAD
    --atomic                                      # Use an atomic transaction to update local refs.
    --depth: int                                  # Limit fetching to n commits from the tip
    --deepen: int                                 # Limit fetching to n commits from the current shallow boundary
    --shallow-since: string                       # Deepen or shorten the history by date
    --shallow-exclude: string                     # Deepen or shorten the history by branch/tag
    --unshallow                                   # Fetch all available history
    --update-shallow                              # Update .git/shallow to accept new refs
    --negotiation-tip: string                     # Specify which commit/glob to report while fetching
    --negotiate-only                              # Do not fetch, only print common ancestors
    --dry-run                                     # Show what would be done
    --write-fetch-head                            # Write fetched refs in FETCH_HEAD (default)
    --no-write-fetch-head                         # Do not write FETCH_HEAD
    --force(-f)                                   # Always update the local branch
    --keep(-k)                                    # Keep dowloaded pack
    --multiple                                    # Allow several arguments to be specified
    --auto-maintenance                            # Run 'git maintenance run --auto' at the end (default)
    --no-auto-maintenance                         # Don't run 'git maintenance' at the end
    --auto-gc                                     # Run 'git maintenance run --auto' at the end (default)
    --no-auto-gc                                  # Don't run 'git maintenance' at the end
    --write-commit-graph                          # Write a commit-graph after fetching
    --no-write-commit-graph                       # Don't write a commit-graph after fetching
    --prefetch                                    # Place all refs into the refs/prefetch/ namespace
    --prune(-p)                                   # Remove obsolete remote-tracking references
    --prune-tags(-P)                              # Remove any local tags that do not exist on the remote
    --no-tags(-n)                                 # Disable automatic tag following
    --refmap: string                              # Use this refspec to map the refs to remote-tracking branches
    --tags(-t)                                    # Fetch all tags
    --recurse-submodules: string                  # Fetch new commits of populated submodules (yes/on-demand/no)
    --jobs(-j): int                               # Number of parallel children
    --no-recurse-submodules                       # Disable recursive fetching of submodules
    --set-upstream                                # Add upstream (tracking) reference
    --submodule-prefix: string                    # Prepend to paths printed in informative messages
    --upload-pack: string                         # Non-default path for remote command
    --quiet(-q)                                   # Silence internally used git commands
    --verbose(-v)                                 # Be verbose
    --progress                                    # Report progress on stderr
    --server-option(-o): string                   # Pass options for the server to handle
    --show-forced-updates                         # Check if a branch is force-updated
    --no-show-forced-updates                      # Don't check if a branch is force-updated
    -4                                            # Use IPv4 addresses, ignore IPv6 addresses
    -6                                            # Use IPv6 addresses, ignore IPv4 addresses
    --help                                        # Display this help message
  ]

  # Check out git branches and files
  export extern "git checkout" [
    ...targets: string@"nu-complete git branches"   # name of the branch or files to checkout
    --conflict: string                              # conflict style (merge or diff3)
    --detach(-d)                                    # detach HEAD at named commit
    --force(-f)                                     # force checkout (throw away local modifications)
    --guess                                         # second guess 'git checkout <no-such-branch>' (default)
    --ignore-other-worktrees                        # do not check if another worktree is holding the given ref
    --ignore-skip-worktree-bits                     # do not limit pathspecs to sparse entries only
    --merge(-m)                                     # perform a 3-way merge with the new branch
    --orphan: string                                # new unparented branch
    --ours(-2)                                      # checkout our version for unmerged files
    --overlay                                       # use overlay mode (default)
    --overwrite-ignore                              # update ignored files (default)
    --patch(-p)                                     # select hunks interactively
    --pathspec-from-file: string                    # read pathspec from file
    --progress                                      # force progress reporting
    --quiet(-q)                                     # suppress progress reporting
    --recurse-submodules: string                    # control recursive updating of submodules
    --theirs(-3)                                    # checkout their version for unmerged files
    --track(-t)                                     # set upstream info for new branch
    -b: string                                      # create and checkout a new branch
    -B: string                                      # create/reset and checkout a branch
    -l                                              # create reflog for new branch
    --help                                          # Display this help message
  ]

  # Push changes
  export extern "git push" [
    remote?: string@"nu-complete git remotes",      # the name of the remote
    ...refs: string@"nu-complete git branches"      # the branch / refspec
    --all                                           # push all refs
    --atomic                                        # request atomic transaction on remote side
    --delete(-d)                                    # delete refs
    --dry-run(-n)                                   # dry run
    --exec: string                                  # receive pack program
    --follow-tags                                   # push missing but relevant tags
    --force-with-lease: string                      # require old value of ref to be at this value
    --force(-f)                                     # force updates
    --ipv4(-4)                                      # use IPv4 addresses only
    --ipv6(-6)                                      # use IPv6 addresses only
    --mirror                                        # mirror all refs
    --no-verify                                     # bypass pre-push hook
    --porcelain                                     # machine-readable output
    --progress                                      # force progress reporting
    --prune                                         # prune locally removed refs
    --push-option(-o): string                       # option to transmit
    --quiet(-q)                                     # be more quiet
    --receive-pack: string                          # receive pack program
    --recurse-submodules: string                    # control recursive pushing of submodules
    --repo: string                                  # repository
    --set-upstream(-u)                              # set upstream for git pull/status
    --signed: string                                # GPG sign the push
    --tags                                          # push tags (can't be used with --all or --mirror)
    --thin                                          # use thin pack
    --verbose(-v)                                   # be more verbose
    --help                                          # Display this help message
  ]
```
のようになります。

これで`git checkout`と`git push`のときにTABでブランチ名を補完できるようになるはずです。
（諸事情で実際に動いてる環境からのコピペができていません。
そのため、括弧の漏れなどがある可能性があります。
もしそういったものを見つけた場合はコメントで教えていただけると幸いです。）