Move "close tasks on commit" code out of field specification stuff

Summary: Ref T2222. There's some magic here, just port it forward in a mostly-reasonable way. This could use some refinement eventually.

Test Plan: Pushed commits with "Fixes" and "Ref" language, used reparse.php to trigger the new code. Saw expected updates in Maniphest.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8471