Implement "Differential Revision" fields in Herald pre-commit content adapter

Summary: Ref T4195. Allows you to write revision-based commit hooks, e.g. block all commits with no corresponding revision.

Test Plan:
Here's are the fields populating:

{F90989}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7806