compare: allow nil newDh and oldDh
compare: allow nil newDh and oldDh

This allows people to create synthetic InodeDeltas, which is something
that umoci would like to be able to do in order to nicely create 'umoci
insert' layers.

Signed-off-by: Aleksa Sarai <a8eb12d32d6ec07cdf737175ec7d4a0f1b4e1ee7@suse.de>

vbatts/go-mtree,vbatts/go-mtree