dgit: Interoperate with git-debrebase, automatically

* Recognise gdr branches, by the presence of corresponding
  ffq-prev or debrebase-last refs.

* Before trying to do our own multi-patch linear quilt fixup, on a gdr
  branch, see if gdr can do it.  If so that is faster and better.

* If our branch is an unstitched gdr branch, which would be ff from
  the archive if we were to stitch it, then do so.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>