Add recipe for git-rebase-mode.el

git-rebase-mode.el is a major-mode for editing Git rebase files, the
kind produced by git rebase --interactive. It's a part of Magit that
can be used independently, for example by having an emacs server
running and "emacsclient" set as the EDITOR.
Merge pull request #1544 from capitaomorte/master

Add recipe for git-rebase-mode.el