sha1_file.c: make sure packs in an alternate odb is named properly.

sha1_file.c: make sure packs in an alternate odb is named properly.

We somehow ended up registering packs in alternate object
directories as "dir/object//pack/pack-*", which confusd the
update-server-info code very badly.  Also we did not attempt to
detect a mistake of listing the object directory itself as one
of the alternates. This does not lead to incorrect behaviour,
but is simply wasteful, so try to do so when we are trivially
able to.

Signed-off-by: Junio C Hamano <junkio@cox.net>