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