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