notes: extract logic into set_display_notes()

Instead of open coding the logic that tweaks the variables in
`struct display_notes_opt` within handle_revision_opt(), abstract away the
logic into set_display_notes() so that it can be reused.

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

notes.h

@@ -265,6 +265,16 @@ struct display_notes_opt {
  */
 void init_display_notes(struct display_notes_opt *opt);
 
+/*
+ * Set a display_notes_opt to a given state. 'show_notes' is a boolean
+ * representing whether or not to show notes. 'opt_ref' points to a
+ * string for the notes ref, or is NULL if the default notes should be
+ * used.
+ *
+ * Return 'show_notes' normalized to 1 or 0.
+ */
+int set_display_notes(struct display_notes_opt *opt, int show_notes, const char *opt_ref);
+
 /*
  * Load the notes machinery for displaying several notes trees.
  *