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
