Move mercurial commit metadata parsing into a LowLevel query

Summary: Ref T4195. Same as D7793, but for mercurial. (As usual, SVN needs some goofy nonsense instead, so the next diff will just make this field work.)

Test Plan: Ran reparse.php on Git and Mercurial commits, var_dump'd the output and it looked correct.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7795