Cannot call Mmap.get_hash_prefix except pos=0
https://gitlab.com/dailambda/plebeia/-/blob/master/src/mmap.ml#L41
let get_hash_prefix buf pos =
Format.eprintf "%d, %d\n" pos (pos+28); (* 4, 32 *)
let s = copy buf pos 28 in
Format.eprintf "%d\n" (String.length s); (* 32 *)
Hash.Prefix.of_string s (* Raised exception! *)