Add a shortcut to extend the branches to be rebased with the current
branch without having to type its name.

Change-Id: If2fa117a6418590cdec5e65430f65dcdc825b611
Reviewed-on: https://chromium-review.googlesource.com/c/1483030Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>