commit-graph.c: extract 'refs_cb_data'

commit-graph.c: extract 'refs_cb_data'

In subsequent patches, we are going to update a progress meter when
'add_ref_to_set()' is called, and need a convenient way to pass a
'struct progress *' in from the caller.

Introduce 'refs_cb_data' as a catch-all for parameters that
'add_ref_to_set' may need, and wrap the existing single parameter in
that struct.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>