fix(checkpoint): retry a backfill on a ref that appeared while it probed

A backfill to a pre-switch checkpoint asks the remote for its ref, and a plain git fetch from another process can install that ref meanwhile, since fetch takes no ref lock. The copy then saw the ref and declined, and the backfill fell through to v1, hidden behind the ref and never pushed. The copy now reports the existing ref so the backfill retries there. A retry that still finds nothing falls through as before.