Improve DifferentialRevisionQuery and add the ability to query by arcanist project

Summary:

• We currently post-filter by branches, but should do this in SQL. See T799.
• We currently identify branch-name-matches as being in the working copy even if they belong to a different project (e.g., two different projects with commits on the branch "master"). See T1100.
• Denormalize branch and project information into DifferentialRevision.
• Expose project information in the API.

Test Plan: Ran conduit API queries with branches and arc project IDs, got reasonable results.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1100, T799

Differential Revision: https://secure.phabricator.com/D2190