Move git commit metadata parsing into a LowLevelQuery

Summary: Ref T4195. I need to issue this command from the pre-commit hook to get commit bodies for hooks.

Test Plan: Ran reparse.php --message --trace and dumped the $ref, which looked correct.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7793