Herald - fix change type bug

Summary: wasn't working due to some type issues. Fixes T4756. I also made it display nicer while I was debugging this.

Test Plan: created a herald rule to block changes that added refs. git tag -a "test" -m "test test"; git push origin test got me blocked!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4756

Differential Revision: https://secure.phabricator.com/D8724