upload-pack.c: do not pass confusing cb_data to mark_our_ref()

The callee does not use cb_data, and the caller is an intermediate
function in a callchain that later wants to use the cb_data for its
own use.  Clarify the code by breaking the dataflow explicitly by
not passing cb_data down to mark_our_ref().

Signed-off-by: Junio C Hamano <gitster@pobox.com>