Add didParseCommit() to DifferentialFieldSpecification

Summary:
This is mostly ripped from D2721, but doesn't implement the T945 part.

After we parse a commit message, give DifferentialFieldSpecifications an opportunity to react to the message as well (e.g., by updating related objects).

Test Plan: Impelmented a var_dump() body, ran reparse.php on a commit.

Reviewers: vrana, 20after4, btrahan, edward

Reviewed By: 20after4

CC: aran

Maniphest Tasks: T945, T1544

Differential Revision: https://secure.phabricator.com/D3444