Only count the size of each hard link once for each directory

The displayed directory sizes are now fully correct, although in its
current state it's not all that intuitive because:

  directory size != sum(size of all files and subdirectories)

This should probably be fixed later on by splitting the sizes into a
shared and non-shared part.

Also, the sizes displayed after a recalculation or deletion are
incorrect, I'll fix this later on.