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".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4768, T11179

Differential Revision: https://secure.phabricator.com/D16164