Summary: Fixes T5788. We already have this as a pre-commit field, add it as a post-commit field too.
Test Plan: Ran this rule on a merge commit. Also ran it on a non-merge commit. Both got the correct value.
Reviewers: avivey, chad
Reviewed By: avivey, chad
Subscribers: avivey
Maniphest Tasks: T5788
Differential Revision: https://secure.phabricator.com/D14685
