pack-redundant: delay creation of unique_objects

pack-redundant: delay creation of unique_objects

Instead of initializing unique_objects in `add_pack()`, copy from
all_objects in `cmp_two_packs()`, when unwanted objects are removed from
all_objects.

This will save memory (no allocate memory for alt-odb packs), and run
`llist_sorted_difference_inplace()` only once when removing ignored
objects and removing objects in alt-odb in `scan_alt_odb_packs()`.

Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>