pretty printing: extend %G? to include 'N' and 'U'

Expand %G? in pretty format strings to 'N' in case of no GPG signature
and 'U' in case of a good but untrusted GPG signature in addition to
the previous 'G'ood and 'B'ad. This eases writing anyting parsing
git-log output.

Signed-off-by: Sebastian Götte <jaseg@physik-pool.tu-berlin.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -1145,6 +1145,8 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 			switch (c->signature_check.result) {
 			case 'G':
 			case 'B':
+			case 'U':
+			case 'N':
 				strbuf_addch(sb, c->signature_check.result);
 			}
 			break;