Added support for --branch to git p4 rebase in git-p4.py

[Samuel Hishmeh <samuel.hishmeh@xxxxxxxxx>]

I added support for the --branch option to the git p4 rebase command in
git-p4.py. Here's the commit:

https://github.com/git/git/commit/27b68a48969ee9be091abfd971e0c0aa092b1638

Here's the pull request:

https://github.com/git/git/pull/1759

Here's the patch: