Modularize all Diffusion commit Herald fields

Summary: Ref T8726.

Test Plan:
Created a giant rule with every commit field:

Ran the upgrade, got the same rule with new fields:

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