R1:8476ad1a281c

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…