Teach git-branch to delete tracking branches with -r -d

Teach git-branch to delete tracking branches with -r -d

Because -r already means "remote" when listing, you can say:

	$ git branch -d -r origin/todo origin/html origin/man

I just twisted it not to check fast-forwardness with the current
branch when you are removing a tracking branch.  Most likely,
removal of a tracking branch is not because you are "done with"
it (for a local branch, it usually means "you merged it up"),
but because you are not even interested in it.  In other words,
remote tracking branches are more like tags than branches.

Signed-off-by: Junio C Hamano <junkio@cox.net>