Modernize "PhabricatorRepositoryPushLogSearchEngine"

Summary: Depends on D18917. Ref T13046. While I'm in here, update this to use more modern construction.

Test Plan: Browed and queried for push logs.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13046

Differential Revision: https://secure.phabricator.com/D18918