Make "purge_cache.php --differential" also purge the inline comment cache

Summary: We purge the cache for primary comments only right now, not inline
comments.

Test Plan: Ran "purge_cache.php --differential".

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T683

Differential Revision: https://secure.phabricator.com/D1451