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>
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,