gitlab-mirror: better handling of stacked topic cases
This is akin yet a bit different than heptapod#43 (closed). With hg-evolve 9.3.1, the server won't consider some obsolete topic heads in its protection against multiple heads. These can in turned be listed in our branchmap, even though it's done with the 'served' filter.
A typical case is when two topics are stacked and the head of the first one is amended.
We use directly the same function as evolve does to filter the branchmap view on our side. This makes the scenarios of heptapod-tests!30 (merged) pass.
Edited by Georges Racinet