git-pack-redundant: speed and memory usage improvements

Slab allocation of llist entries gives some speed improvements.

Not computing the pack_list permutaions all at once reduces memory
usage greatly on repositories with many packs.

Signed-off-by: Lukas Sandström <lukass@etek.chalmers.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>