Diffusion / Herald - tweak "Accepted Differential Revision exists" Herald field
Summary: make it use the value of the revision before any post-commit magic has occurred. Fixes T4754
Test Plan: made a herald rule that said "if revision exists, and revision accept does not exists, block push". tried to push a commit that had a revision that wasn't accepted and I was blocked.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: mbishopim3, epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D10393