R1:014a873773eb

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…