Recognize the official "Go" magic regexp for generated code as generated

Summary: See PHI1112. See T784. Although some more general/flexible solution is arriving eventually, adding this rule seems reasonable for now, since it's not a big deal if we remove it later to replace this with some fancier system.

Test Plan: Created a diff with the official Go generated marker, saw the changeset marked as generated.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20237