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
