git-debrebase convert-from-gbp: Make it ff of dgit view, if we can.

git-debrebase convert-from-gbp: Make it ff of dgit view, if we can.

Look for dgit-generated tags so we can usually make the new branch ff
of the dgit view.

Specifically, we fish the most recent released version out of
debian/changelog and look for a DISTRO/VSN tag and a corresponding
archive/DISTRO/VSN tag.  If this all looks good, we declare ff of
archive/.

This should avoid the need for dgit push --overwrite in most cases.

We do not check the DISTRO name.  In principle dgit could find the
distro name from the suite name, but that makes the software layering
quite complicated.  (For example, we would have to provide a way to
tell gdr what program to run for dgit, for the benefit of the test
suite.)  I think the current approach is good enough; there are not
likely to be any tag pairs that look like the right kind of thing but
are in fact something else entirely, so we're not likely to
accidentally trash some wanted changes.

Closes:#903132.

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