Create object subdirectories on demand (phase II)

This removes the unoptimization.  The previous round does not mind
missing fan-out directories, but still makes sure they exist, lest
older versions choke on a repository created/packed by it.

This round does not play that nicely anymore -- empty fan-out
directories are not created by init-db, and will stay removed by
prune-packed.  The prune command also removes empty fan-out directories.

Signed-off-by: Junio C Hamano <junkio@cox.net>