Depends on D20208. Ref T13255. See that task for some long-winded discussion and rationale. Short version:
- This is a list of operations instead of a list of old/new PHIDs because of scalability issues for large lists (T13056).
- This is a fairly verbose list (instead of, for example, the more concise internal map we sometimes use with "+" and "-" as keys) to try to make the structure obvious and extensible in the future.
- The "add" and "remove" echo the *.edit operations.