Add activeDiffPHID to differential.query

Summary: Ref T4809. This saves us a few round trips to find a Buildable, and generally makes the notion of "active" more explicit (i.e., not just the diff with the largest ID). In the future, we may let you revert to previous diffs, which would make the "largest number" rule not always correct.

Test Plan: Ran differential.query, got sensible results.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4809

Differential Revision: https://secure.phabricator.com/D8800