pack-objects: don't use too many threads with few objects

pack-objects: don't use too many threads with few objects

If there are few objects to deltify, they might be split amongst threads
so that there is simply no other objects left to delta against within
the same thread.  Let's use the same 2*window treshold as used for the
final load balancing to allow extra threads to be created.

This fixes the benign t5300 test failure.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Tested-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>