Add flags to get_commit_notes() to control the format of the note string

This patch adds the following flags to get_commit_notes() for adjusting the
format of the produced note string:
- NOTES_SHOW_HEADER: Print "Notes:" line before the notes contents
- NOTES_INDENT: Indent notes contents by 4 spaces

Suggested-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -978,7 +978,8 @@ void pretty_print_commit(enum cmit_fmt fmt, const struct commit *commit,
 		strbuf_addch(sb, '\n');
 
 	if (fmt != CMIT_FMT_ONELINE)
-		get_commit_notes(commit, sb, encoding);
+		get_commit_notes(commit, sb, encoding,
+				 NOTES_SHOW_HEADER | NOTES_INDENT);
 
 	free(reencoded);
 }