HgitalyRepository: use FindMergeBase
This is the counterpart of hgitaly#26.
This method is the one behind the computation of
base SHA for Merge Requests.
Since we don't want to swap the RepositoryService as a whole (most of its RPCs are for %HGitaly3: fully in charge), we'll probably can just make a temporary
hgitaly_repository_client method to access the HGitaly service instead of changing the behaviour of the existing (
Writing RSpec tests for the %HGitaly1: Hg SHAs time frame would be a good thing.