RepositoryService: implement FindMergeBase
Most methods of RepositoryService are of the global type (create etc.), but not this one.
This is perhaps the only method we could have been needing for %HGitaly1: Hg SHAs, but I've replaced its call by a conversion to Git and back in the run to working MRs after heptapod#364 (closed).
We won't wait until all methods of RepositoryService are ready for this one to be called.