Separate all commit message field parsing out of Differential custom fields

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