pack-redundant: fix crash when one packfile in repo

pack-redundant: fix crash when one packfile in repo

Command `git pack-redundant --all` will crash if there is only one
packfile in the repository.  This is because, if there is only one
packfile in local_packs, `cmp_local_packs` will do nothing and will
leave `pl->unique_objects` as uninitialized.

Also add testcases for repository with no packfile and one packfile
in t5323.

Reported-by: Daniel C. Klauer <daniel.c.klauer@web.de>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>