upload-pack: move extra_edge_obj to upload_pack_data

upload-pack: move extra_edge_obj to upload_pack_data

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

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

While at it let's properly initialize and clear 'extra_edge_obj'
in the appropriate 'upload_pack_data' initialization and
clearing functions.

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