upload-pack: make want_obj not global

upload-pack: make want_obj not global

Because upload_pack_v2() can be invoked multiple times in the same
process, the static variable want_obj may not be empty when it is
invoked. To make further analysis of this situation easier, make the
variable local; analysis will be done in a subsequent patch.

The new local variable in upload_pack_v2() is static to preserve
existing behavior; this is not necessary in upload_pack() because
upload_pack() is only invoked once per process.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>