Allows ignoring file metadata differences (permissions, ownership)
when comparing files.
This is especially useful on darwin, where we often run into subtle
issues like:
-Device: 1,23 Access: (0444/-r--r--r--) Uid: ( 0/ root) Gid: ( 0/ wheel)
+Device: 1,23 Access: (0444/-r--r--r--) Uid: ( 0/ root) Gid: ( 350/ nixbld)