Update DifferentialDiff: add repositoryPHID, drop parentRevisionID

Summary:
Moves away from ArcanistProjects:

• Adds storage for diffs to be directly associated with a repository (instead of indirectly, through arcanist projects). Not really populated yet.
• Drops parentRevisionID, which is obsoleted by the "Depends On" edge. This is not exposed in the UI anywhere and doesn't do anything. Resolves TODO.

Test Plan: Ran storage upgrades, browsed around, lots of grep.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8072