Be more lenient when accepting "Differential Revision" in the presence of custom ad-hoc commit message fields

Summary:
Fixes T8360. We will now parse revisions out of "Differential Revision: X" followed by other ad-hoc fields which we do not recognize. Previously, these fields would be treated as part of the value.

(In the general case, other fields may line wrap so we can't assume that fields are only one line long. However, we can make that assumption safely for this field.)

Also maybe fix whatever was going on in T9965 although that didn't really have a reproduction case.

Test Plan: Added unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8360

Differential Revision: https://secure.phabricator.com/D17121