Fix Herald "any changed content" rule for commits

Summary: Fixes T1330. This has been broken with a TODO since we open sourced. :/

Test Plan: Used test console to verify this produced the correct field value for several commits.

Reviewers: btrahan, chad, vrana

Reviewed By: chad

CC: aran

Maniphest Tasks: T1330

Differential Revision: https://secure.phabricator.com/D4563