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

Maniphest Tasks: T4754, T4574

Differential Revision: https://secure.phabricator.com/D10393