We read loose and packed rerferences in two steps, but after
deciding to read a loose ref but before actually opening it to read
it, another process racing with us can unlink it, which would cause
us to barf. Update the codepath to retry when such a race is
detected.
* mh/loose-refs-race-with-pack-ref:
resolve_ref_unsafe(): close race condition reading loose refs
resolve_ref_unsafe(): handle the case of an SHA-1 within loop
resolve_ref_unsafe(): extract function handle_missing_loose_ref()