prune_stale_shallow_grafts dropped grafts no live ref points at, but the
remote-tracking reflog still naming a dropped fetch tip made the --reflog
fail-safe walk fail (its parent was never fetched), rolling the prune back
on every run — grafts kept accumulating and the next update fell into
orphan divergence (#124645). Expire exactly the fetch reflogs that pin a
dropped graft before writing the trimmed file; user reflogs (HEAD, local
branches) are never touched, so the fail-safe rollback still guards them.
Fixes#124645
6b51b0fd4e