don't load objects needlessly when repacking

If no delta is attempted on some objects then it is useless to load them
in memory, neither create any delta index for them.  The best thing to
do is therefore to load and index them only when really needed.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>