Move commit hash querying to DiffusionLowLevelCommitQuery

Summary: Ref T4195. I need this for the Herald pre-commit rules, and it generally simplifies things.

Test Plan: Used reparse.php plus var_dump() to inspect refs in Git, Mercurial and SVN repos. They all looked correct and reparsed correctly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7804