upload-pack: move symref to upload_pack_data

upload-pack: move symref to upload_pack_data

As we cleanup 'upload-pack.c' by using 'struct upload_pack_data'
more thoroughly, we are passing around that struct to many
functions, so let's also pass 'struct string_list symref' around
at the same time by moving it from a local variable in
upload_pack() into a field of 'struct upload_pack_data'.

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