-s causes cmp to print nothing for differing files, for use when only the exit status is of interest.
-z compares the file size first, for regular files, and fails the comparison early if they do not match.
Prior to this change -s implied -z as an optimization, but this is not valid when file offsets are specified. Now, enable the -z optimization for -s only if both skip arguments are not provided / 0.
Reported by: William Ahern
Sponsored by: The FreeBSD Foundation