Disable Herald and enormous change protection for repository initial imports

Summary: See PHI514. Ref T13114. Ref T8951. When a push is an "initial import" (a push of at least 7 commits to an empty repository) don't run Herald or enormous change protection.

Test Plan: Pushed some non-initial changes to a repository, and some initial changes.

Maniphest Tasks: T13114, T8951

Differential Revision: https://secure.phabricator.com/D19265