Merge rebase mode from Magit

This merges `rebase-mode.el` from Magit.

@tarsius Shall I merge it?

Btw, I'd like to rename `rebase-mode.el` to `git-rebase-mode.el` for the sake of consistency with other modes, and because I think that we should not claim a generic name such has `rebase-mode`.  After all, you can “rebase” many things, including commits and branches in other DVCS.

I still need to update the credits in README.md to account for rebase mode.  Do you know who contributed to this mode, other than those listed in the file header?