[PATCH] branch: implement shortcut to delete last branch

[Aaron Greenberg <p@aaronjgreenberg.com>]

This patch gives git-branch the ability to delete the previous
checked-out branch using the "-" shortcut. This shortcut already exists
for git-checkout, git-merge, and git-revert. One of my common workflows
is to do some work on a local topic branch and push it to a remote,
where it gets merged in to 'master'. Then, I switch back to my local
master, fetch the remote master, and delete the previous topic branch.

$ git checkout -b topic-a
$ # Do some work...
$ git commit -am "Implement feature A"
$ git push origin topic-a

# 'origin/topic-a' gets merged into 'origin/master'

$ git checkout master
$ git branch -d topic-a
$ # With this patch, a user could simply type
$ git branch -d -

I think it's a useful shortcut for cleaning up a just-merged branch
(or a just switched-from branch.)