R1:a6fbde784c71

Modernize "PhabricatorRepositoryPushEventQuery"

Summary: Depends on D18914. Updates this Query to use slightly more modern construction while I'm working in adjacent code.

Test Plan: Viewed push logs in web UI.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18915