Move commit message/metadata field query to a separate class

Summary: Ref T4195. I need to query commit metadata to figure out which revision a commit is associated with. Move this out of the MessageParser so the code can be called from the HookEngine.

Test Plan: Used reparse.php to reparse a variety of SVN, Mercurial and Git commits. Used var_dump() to verify sensible fields were returned.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7805