Modularize all Diffusion commit Herald fields
Summary: Ref T8726.
Test Plan:
Created a giant rule with every commit field:
{F594686}
Ran the upgrade, got the same rule with new fields:
{F594688}
Used "Test Console" to run transcripts, saw all the fields populate correctly.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T8726
Differential Revision: https://secure.phabricator.com/D13567