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.c

diff --git a/notes.c b/notes.c
index fd6cef1..53d1e77 100644
--- a/notes.c
+++ b/notes.c
@@ -1039,6 +1039,12 @@ struct notes_tree **load_notes_trees(struct string_list *refs, int flags)
 	return trees;
 }
 
+void init_display_notes(struct display_notes_opt *opt)
+{
+	memset(opt, 0, sizeof(*opt));
+	opt->use_default_notes = -1;
+}
+
 void load_display_notes(struct display_notes_opt *opt)
 {
 	char *display_ref_env;

notes.h

diff --git a/notes.h b/notes.h
index 1ce5284..c0b7123 100644
--- a/notes.h
+++ b/notes.h
@@ -261,6 +261,11 @@ struct display_notes_opt {
 };
 
 /*
+ * 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.
  *
  * If 'opt' is not NULL, then it specifies additional settings for the

revision.c

diff --git a/revision.c b/revision.c
index d4aaf0e..24ad974 100644
--- a/revision.c
+++ b/revision.c
@@ -1637,7 +1637,7 @@ void repo_init_revisions(struct repository *r,
 		revs->diffopt.prefix_length = strlen(prefix);
 	}
 
-	revs->notes_opt.use_default_notes = -1;
+	init_display_notes(&revs->notes_opt);
 }
 
 static void add_pending_commit_list(struct rev_info *revs,