ssh signing: make verify-tag consider key lifetime

Set the payload_type for check_signature() when calling verify-tag.
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>

tag.c

@@ -25,6 +25,7 @@ static int run_gpg_verify(const char *buf, unsigned long size, unsigned flags)
 		return error("no signature found");
 	}
 
+	sigc.payload_type = SIGNATURE_PAYLOAD_TAG;
 	sigc.payload = strbuf_detach(&payload, &sigc.payload_len);
 	ret = check_signature(&sigc, signature.buf, signature.len);