Simplify index access condition in count-objects, pack-redundant

Simplify index access condition in count-objects, pack-redundant

My earlier lazy index opening patch changed this condition to check
index_data and call open_pack_index if it was NULL. In truth we only
care about num_objects.  Since open_pack_index does no harm if the
index is already open, and all indexes are likely to be closed in
this application, the "performance optimization" of inlining the
index_data check here was wrong.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>