Revert "git-commit-insert-header-as-self: stop using some env vars"

From the git-commit-tree(1) man page:
  #+begin_quote
    While parent object ids are provided on the command line,
    author and committer information is taken from the following
    environment variables, if set:

        GIT_AUTHOR_NAME
        GIT_AUTHOR_EMAIL
        GIT_AUTHOR_DATE
        GIT_COMMITTER_NAME
        GIT_COMMITTER_EMAIL
        GIT_COMMITTER_DATE
        EMAIL

    In case (some of) these environment variables are not set,
    the information is taken from the configuration items
    user.name and user.email, or, if not present, system user
    name and the hostname used for outgoing mail (taken from
    /etc/mailname and falling back to the fully qualified
    hostname when that file does not exist).
  #+end_quote

This reverts commit 61a6e94d399dd53fe0a56dd12bbe7184f9915b48.

Signed-off-by: Pieter Praet <pieter@praet.org>
`git-rebase-mode': change name to "Git Rebase"

To be consistent with `git-commit-mode', which is called "Git Commit".

Signed-off-by: Pieter Praet <pieter@praet.org>
`git-rebase-mode-map': fix docstring typo

Signed-off-by: Pieter Praet <pieter@praet.org>
`git-rebase-mode-menu': make "Done" call `git-rebase-server-edit'

Commit ba6a5450 bound "q" and "C-c C-c" to `git-rebase-server-edit'
instead of plain `server-edit' in `git-rebase-mode-map', but didn't
update `git-rebase-mode-menu' accordingly.

I'd be surprised if anyone is actually using the menu for anything but
discovering available functionality (or even has it enabled, for that
matter), but still, it's better to be consistent.

Signed-off-by: Pieter Praet <pieter@praet.org>
`git-rebase-move-line-up': don't swap top lines

When running `git-rebase-move-line-up' on a line that is already at the
top of the list, the first and second lines are transposed, so the line
we meant to move *up* is effectively moved *down*.

Prevent this by only transposing lines if point isn't at the beginning
of the buffer.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #41 from praet/rebase-mode

git-rebase-mode fixes
Merge pull request #42 from praet/commit-insert-header

Revert "git-commit-insert-header-as-self: stop using some env vars"