Summary:
In order to move our hg-git mirroring off of the main hg servers, we
need to make it possible for the hg servers to compute the hg-git mapping
without having the entire git repository available. To do so, let's store the
git hash as an extra in the hg commit.
This breaks bidirectionality, but we've long since not needed that.
Reviewed By: phillco
Differential Revision: D13362980
fbshipit-source-id: 51df709bc5e77d78bb963abf90d0c35bb743d966