Summary:
We need to store relation between Hg changesets and Bonsai changesets.
- `BonsaiHgMapping` is exactly this mapping which establishes injective relation between `{Hg|Bonsai}Changeset`
Reviewed By: StanislavGlebik
Differential Revision: D8801254
fbshipit-source-id: c7df14172e6c2d67c039a24e1bb821e6d92860af