Use DiffQuery instead of direct hunk loads in MessageParser daemon

Summary: Ref T5179. Ref T4045. Continue reducing the number of direct hunk loads we perform.

Test Plan: Pushed a closing commit, used scripts/repository/reparse.php --message ... to trigger this logic, got a sensible/accurate result.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4045, T5179

Differential Revision: https://secure.phabricator.com/D9288