So it would be possible to write a `git-debrebase' tool which would
take (for example) B4, above, and be able to perform functions like:

 * Strip pseudomerges: Rewrite the current branch so it contains no
   pseudomerges, turning ...B3 into ...@-A-1-2-B3.  (This should
   make a note, in your .git/ somewhere, of the original branch
   tip so that it can be overwritten with a pseudomerge.)

 * Cleanup branch: Reorganise the current branch so that the debian/
   changes come first, turning -@-A-1-2-B3 into ...@-A-B-1-2-3.

 * New upstream rebase: Start rebasing onto a new upstream version,
   turning ...#..@-A-B-1-2-3 into (...#..@-A-B-|...#'-)@'-1-2.  This
   would be a wrapper around git-rebase, which prepares @' and then
   tries to rebase 1 2 onto @'.  So if you ask for an interactive
   rebase @' doesn't appear in your commit list.

   Note that the construction of @' cannot fail because @' simply
   copies debian/ from B and and everything else from #'.  (Rebasing A
   and B is undesirable.  We want the debian/ files to be non-rebasing
   so we can `git log' and get the packaging history.)

 * Record pseudomerge.  This is like "committing" your patch queue.
   The LH parent is taken from the previously recorded tip.  (We could
   perhaps check that this is consistent with what we see in
   debian/changelog, but that is not a sufficient check so the
   recorded tip check is primary.)

Maybe some of these operations should automatically edit
debian/changelog.