git_prompt: optionally show upstream branch name

Adds a "name" option to __fish_git_prompt_showupstream that shows an
abbreviated branch name when the upstream type is verbose.

Based on git.git 1f6806c: git-prompt.sh: optionally show upstream
branch name
git_prompt: Add upstream_prefix when verbose

git.git's git-prompt may not contain a configurable prefix, but it
does display a space before the upstream information when displaying
verbose information.  Rather than using a space always or never,
default to a space whenever verbose is in showupstream.
git_prompt: Allow all set_color arguments

There is no need to explicitly check for two arguments and set --bold.
Instead the user can simply "set __fish_git_prompt_color_flags --bold
red".

The current check violates the expectation set by the documentation
that you can use any set_color argument as the current code interprets
"--bold red" as "--bold --bold" instead.

Plus, by passing the full contents of the variable directly, the user
can do more adventurous things like set the background as well.