Herald - make dry runs work for "apply once" rules after they have been applied

Summary: Fixes T3719

Test Plan: https://secure.phabricator.com/T3719#comment-7

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3719

Differential Revision: https://secure.phabricator.com/D6968