`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