notes: refactor display notes extra refs field

There's no need to use an extra pointer, which just ends up
leaking memory. The fact that the list is empty tells us the
same thing.

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

notes.h

@@ -1,6 +1,8 @@
 #ifndef NOTES_H
 #define NOTES_H
 
+#include "string-list.h"
+
 /*
  * Function type for combining two notes annotating the same object.
  *
@@ -257,7 +259,7 @@ struct string_list;
 
 struct display_notes_opt {
 	unsigned int suppress_default_notes:1;
-	struct string_list *extra_notes_refs;
+	struct string_list extra_notes_refs;
 };
 
 /*