Tag rename unfinished business
(things resulting from searching for `tag' where searching for
 `debiantag' wouldn't find them)

 - protocol version 4
      implies ability to sign new tag (this is already there
         so simply advertising version 4 will work)
      needs also new ability to sign old-format tag in split
         brain way

 - many actual tests

 - other tag should be [dgit Maintainer-Tag quilt=$quilt-mode]

 - in dopush (?), we need to sort out synthetic fast forard
    this should not depend entirely on quilt mode
    but will always be needed if quiltmode_splitbrain

 - there is some confusion between $split_brain (did we actually
   generate any commits) and quiltmode_splitbrain (might we have done,
   including in previous pushes); search for $split_brain and check
   that use sites are correct


Want tests for
 - start repo with old tag, push with new tag, run
   "old" dgit to push again with new tag, push again with
   old tag
 - rejecting push of same version with one tag when then the other

That, where appropriate, maint view tags are pushed to dgit git repo

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Provide --gpbpq[=:] options
Document --quilt=unapplied|gbp|dpm