R1:4bbe6f307a7a

Resolve relationship edit conflicts more naturally

Summary:
Ref T11179. Ref T4768. Currently, on `master`, if two users open "Edit Revisions" at the same time, then add revisions A and B, only the last state wins (just "B").

Instead, apply these as "add A" and "add B" so they merge in a natural way.

Test Plan:
- Opened edit dialog in two windows.
- Added "A" in one, "B" in the other.
- Saved both.
- Saw "Added A" and "Added B" transactions, instead of "Added A" and "Removed A, added B".