Summary:
Ref T13639. Make schema changes:
- Make repositoryID nullable, for revisions with no repository.
- Remove "epoch", which has no readers and no clear use.
- Change the ordering of the key, since "pathID" has more unique values and no queries ever issue without it.
Test Plan:
- Ran `bin/storage upgrade`, got a clean schema.
- Reindexed all revisions with an external script.
- Reviewed index via debug UI, saw appropriate index for non-repositoy revisions.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13639
Differential Revision: https://secure.phabricator.com/D21617
