Don't run Herald build rules when Differential revisions are updated automatically

Summary: Ref T2543. After D18731, Herald build rules run more often, but now incorrectly try to run builds when Diffusion closes a revision because a commit landed.

Test Plan: Made some mundane updates locally; this is tricky to test comprehensively locally so I'm mostly planning to just push it to secure.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18745