Assigns negative sizes to directories in order to have them listed reliably
before any zero-sized files. That order is what most users expect when
sorting by size.

As side effect directories will appear before files on all filesystem
implementations. To give an example: before this change directories had a size
of 4 KiB when using Linux with ext4 or tmpfs, and with ZFS a size resembling
an estimation of the number of leaves within said directory.