R1:bd8071bb3350

Add a `(dst, type, src)` key to Differential's edge table

Summary:
Ref T1279. This came to me in a dream.

The existing `differential_relationship` table has an `(objectPHID, type)` column, which theoretically is useful for queries like "revisions with X as a reviewer". In practice, I'm not sure it gets used much, but I can get it to show up in at least some query plans.

Add a similar index to the `edge` table. This sequences //before// D7227, which actually migrates the data.

Test Plan:
- Ran…