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