gpg-interface.c: support getting key fingerprint via %GF format

Support processing VALIDSIG status that provides additional information
for valid signatures.  Use this information to propagate signing key
fingerprint and expose it via %GF pretty format.  This format can be
used to build safer key verification systems that verify the key via
complete fingerprint rather than short/long identifier provided by %GK.

Signed-off-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -1256,6 +1256,10 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			if (c->signature_check.key)
 				strbuf_addstr(sb, c->signature_check.key);
 			break;
+		case 'F':
+			if (c->signature_check.fingerprint)
+				strbuf_addstr(sb, c->signature_check.fingerprint);
+			break;
 		default:
 			return 0;
 		}