Summary: Ref T11114. See that task for some discussion. Overall, Differential custom fields ended up with too many responsibilities. Later work in EditEngine provides a more promising model for achieving modularity with smaller, more consistent components. In particular, we have some custom fields like `DifferentialGitSVNIDField` and `DifferentialConflictsField` which serve //only// to support the field parser. This starts pulling commit message responsibilities out of the core list of custom fields and into simpler dedicated parsers. Test Plan: Created and edited revisions from the CLI. Added a bit of test coverage. Reviewers: chad Reviewed By: chad Maniphest Tasks: T11114 Differential Revision: https://secure.phabricator.com/D17058