notes: create init_display_notes() helper

We currently open code the initialization for revs->notes_opt. Abstract
this away into a helper function so that the logic can be reused in a
future commit.

This is slightly wasteful as we memset the struct twice but this is only
run once so it shouldn't have any major effect.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes.h

@@ -260,6 +260,11 @@ struct display_notes_opt {
 	struct string_list extra_notes_refs;
 };
 
+/*
+ * Initialize a display_notes_opt to its default value.
+ */
+void init_display_notes(struct display_notes_opt *opt);
+
 /*
  * Load the notes machinery for displaying several notes trees.
  *