git-review: save reference to mailed revision using tag

Git mail in the branch named work tags the revision that just got mailed
as work.mailed, overwriting the tag if one already exists. This will let you
run things like

	git diff work.mailed

to see what you've changed since the last time you did a git mail.

The reflog will store the tag history, so you can also diff against older
mailed copies:

	git diff work.mailed@{2}
	git diff work.mailed work.mailed@{2}

Fixes golang/go#9293.

Change-Id: Ifae4c4601c6c9641db88383d52a021087b806e74
Reviewed-on: https://go-review.googlesource.com/1444
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>