Fallback to hg-git mapping for migrated native Mercurial repositories

We'll never be 100% sure in #420 (closed) that we converted all references to Git hashes into Mercurial ones.

The mere possibility that upstream could create new columns pointing to Git hashes while we're implementing the migration is a good potential source of misses on our part.

But it's easy to keep the the hg-git map file for migrated repos, and it should be possible to implement a fallback that would be low level enough to catch everything.

This can be experimented with way before #420 (closed) is complete, and would provide us with interesting hints in any case.