pretty: prepare notes message at a centralized place

Instead of passing a boolean show_notes around, pass an optional
string that is to be inserted after the log message proper is shown.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -86,7 +86,7 @@ struct pretty_print_context {
 	enum date_mode date_mode;
 	unsigned date_mode_explicit:1;
 	int need_8bit_cte;
-	int show_notes;
+	char *notes_message;
 	struct reflog_walk_info *reflog_info;
 	const char *output_encoding;
 };