R1:01ea84029d88

Update table schema for "AffectedPath" table

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…