Make searching by tags work in Phriction

Summary: Fixes T12860. Some joins were being dropped because we didn't call parent::...

Test Plan:

• Tagged a document.
• Searched for documents with that tag.
• Before change: got all documents.
• After change: got only tagged documents.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12860

Differential Revision: https://secure.phabricator.com/D18145