This makes use of the generic method of listing bookmarks and tags, so
other extensions that add other namespaces will get their names added
too.
This does mean that bookmarks will come before tags, just like we
apparently decided to order them in the "hg log" output. It doesn't
seem like people would be parsing the rebase output anyway. We also
did 79ab5369d55a (rebase: use _ctxdesc in one more place, 2017-08-29)
recently, so now seems like a good time.