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