Modernize metamta.differential.patch-format

Summary: Change metamta.differential.patch-format over to an enum option now that they're implemented.

Test Plan: Looked at settings page.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D16032