ssh signing: make fmt-merge-msg consider key lifetime

Set the payload_type for check_signature() when generating merge messages to
verify merged tags signatures key lifetimes.
Implements the same tests as for verify-commit.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fmt-merge-msg.c

@@ -533,6 +533,7 @@ static void fmt_merge_msg_sigs(struct strbuf *out)
 		else {
 			buf = payload.buf;
 			len = payload.len;
+			sigc.payload_type = SIGNATURE_PAYLOAD_TAG;
 			sigc.payload = strbuf_detach(&payload, &sigc.payload_len);
 			if (check_signature(&sigc, sig.buf, sig.len) &&
 			    !sigc.output)