Make DiffusionQuery extend PhabricatorQuery

Summary: Ref T2683. Provides access to `formatWhereClause()`, etc. I plan to eventually unify the various DiffusionQuery execute methods as well, which all have special names right now (`loadModifiedPaths()`, `loadWhateverBlah()`, etc).

Test Plan: Ran EverythingImplemented test. Browsed Diffusion.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D5251