Fix "not(project)" in Diffusion

Summary: Fixes T8105. We weren't including the parent WHERE clause fragment correctly.

Test Plan: Used not(project) in a query.

Reviewers: btrahan, avivey

Reviewed By: avivey

Subscribers: avivey, epriestley

Maniphest Tasks: T8105

Differential Revision: https://secure.phabricator.com/D12746