upload-pack: move filter_capability_requested to upload_pack_data

upload-pack: move filter_capability_requested to upload_pack_data

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

It is only used by protocol v0 code since protocol v2 assumes
certain baseline capabilities, but rolling it into
upload_pack_data and just letting v2 code ignore it as it does
now is more coherent and cleaner.

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