Add and use a function for loading the entries of a loose object
subdirectory for a given object ID. It frees callers from deriving the
fanout key; they can use the returned oid_array reference for lookups or
forward range scans.
Suggested-by: Jeff King <peff@peff.net> Signed-off-by: Rene Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>