comparators/directory.py: ignore st_size for directories

This change fixes false negative stat comparison for directories.

Directory sizes are arbitrary, file system dependent, and may differ, depending on the history of the directory.

E.g. see https://github.com/NixOS/nixpkgs/issues/393375

Not trivial to reproduce, but I assume this can be tested if disorderfs had an option to set random st_size for directories.