commit: implement commit_list_contains()

It can be helpful to check if a commit_list contains a commit. Use
pointer equality, assuming lookup_commit() was used.

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

commit.c

@@ -544,6 +544,17 @@ struct commit_list *commit_list_insert(struct commit *item, struct commit_list *
 	return new_list;
 }
 
+int commit_list_contains(struct commit *item, struct commit_list *list)
+{
+	while (list) {
+		if (list->item == item)
+			return 1;
+		list = list->next;
+	}
+
+	return 0;
+}
+
 unsigned commit_list_count(const struct commit_list *l)
 {
 	unsigned c = 0;