[PATCH v2] branch: implement shortcut to delete last branch

I updated the commit message to include my first email's cover letter and cleaned up the test.

Copying Junio, since he also had good comments in the conversation you linked.

I can appreciate Matthieu's points on the use of "-" in destructive commands. As of this writing, git-merge supports the "-" shorthand, which while not destructive, is at least _mutative_. Also, "git branch -d" is not destructive in the same way that "rm -rf" is destructive since you can recover the branch using the reflog.

One thing to consider is that approval of this patch extends the implementation of the "-" shorthand in a piecemeal, rather than consistent, way (implementing it in a consistent way was the goal of the patch set you mentioned in your previous email.) Is that okay? Or is it better to pick up the consistent approach where it was left?