ref-filter: implement '--points-at' option

In 'tag -l' we have '--points-at' option which lets users
list only tags of a given object. Implement this option in
'ref-filter.{c,h}' so that other commands can benefit from this.

This is duplicated from tag.c, we will eventually remove that
when we port tag.c to use ref-filter APIs.

Based-on-patch-by: Jeff King <peff@peff.net>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.h

@@ -42,6 +42,7 @@ struct ref_array {
 
 struct ref_filter {
 	const char **name_patterns;
+	struct sha1_array points_at;
 };
 
 struct ref_filter_cbdata {