Make Herald have "exists" and "not exists" options for differential reviewers on commit rules
Summary: Fixes T1485.
Test Plan: made a herald rule for "not exists". committed to master with no diff. audit was triggered
Reviewers: epriestley
Reviewed By: epriestley
CC: Korvin, aran
Maniphest Tasks: T1485
Differential Revision: https://secure.phabricator.com/D6964