Simplify UNION/ORDER query construction in DifferentialRevisionQuery

Summary: Ref T12680. Use the slightly sleeker construction from D18722 in Differential.

Test Plan: Viewed revision list, reordered by date modified.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12680

Differential Revision: https://secure.phabricator.com/D18727