Implement directory refresh

This complicated the scan code more than I had anticipated and has a
few inherent bugs with respect to calculating shared hardlink sizes.

Still, the merge approach avoids creating a full copy of the subtree, so
that's another memory usage related win compared to the C version.
On the other hand, it does leak memory if nodes can't be reused.

Not quite as well tested as I should have, so I'm sure there's bugs.