Add "Is merge commit?" to commit fields in Herald

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