Implement DiffusionMercurialContainsQuery in Diffusion

Summary:
Currently, we have no implementation, so all Mercurial commits show "None" for "Branches".

Instead, implement this method.

Test Plan: {F34076}

Reviewers: DurhamGoode, vrana, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5145