The current CI pipeline has one job to merge from one downstream to one
upstream (e.g. debian/buster to apertis/v2020dev0).  While that's
reasonable it's not unlikely quite a few package will be on the same
versions between our different distribution release based on the same
upstream which causes a lot of redundant work as well as merge request
for the same content but different git history.

To solve that add a new helper tool which takes an upstream branch and a
list of downstream branches, keeps track of what git reference each is
at and only generates new actual merges for that are different. If two
or more downstreams have the same state then only one merge is done and
multiple merge requests are sent.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>