ref-filter: move ref_cbdata definition into ref-filter.c

This is an implementation detail of how filter_refs() works,
and does not need to be exposed to the outside world. This
will become more important in future patches as we add new
private data types to it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -1476,6 +1476,11 @@ enum contains_result {
 	CONTAINS_YES = 1
 };
 
+struct ref_filter_cbdata {
+	struct ref_array *array;
+	struct ref_filter *filter;
+};
+
 /*
  * Mimicking the real stack, this stack lives on the heap, avoiding stack
  * overflows.