Allow buildables to be queried by container and underlying buildable

Summary: Ref T1049. Adds "Repository", "Revision", "Diff" and "Commit" as searchable fields.

Test Plan: Used all the fields to filter things.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7823