R1:93cb6e3bdeb0

Make updating a revision with the same active diff a no-op

Summary: Ref T13114. See PHI515. Updating a revision with the same, currently active diff became an error at some point (probably D19175). This is inconsistent; make it an allowable no-op instead.

Test Plan:
- Updated a revision's diff via Conduit.
- Updated to the same diff, no-op.
- Tried to update a different revision, error ("already attached elsewhere").
- Updated with a different diff.
- Tried to update with the original diff…