The adhoc mac and dev-channel Windows builds vet the requested ref by
mirroring every branch and tag of this repo into a scratch bare repo and
proving the commit is reachable. Both runner disks are case-insensitive,
and the repo now has two branches differing only in casing, so the files
backend refuses the fetch outright — the whole job dies before checkout.
reftable keys refs in a table rather than as file paths, so both refs
store and every ref stays in the reachability set.