This let diff_delta() abort early if it is going to bust the given
size limit.  Also, only objects larger than 20 bytes are considered
as objects smaller than that are most certainly going to produce
larger deltas than the original object due to the additional headers.
Signed-off-by: NNicolas Pitre <nico@fluxnic.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>