Return attached hashes from differential.query

Summary: See T693. To do "arc branch" performantly in immutable history repositories, we need to be able to do a single query to identify revisions related by hash. Return hash information to enable this.

Test Plan: Ran differential.query.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T693

Differential Revision: https://secure.phabricator.com/D2859