upload-pack: move pack_objects_hook to upload_pack_data

upload-pack: move pack_objects_hook to upload_pack_data

As we cleanup 'upload-pack.c' by using 'struct upload_pack_data'
more thoroughly, let's move the 'pack_objects_hook' static
variable into this struct.

It is used by code common to protocol v0 and protocol v2.

While at it let's also free() it in upload_pack_data_clear().

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>