tag: mark "--message" option with NONEG

We do not allow "--no-message" to work now, as the option callback returns "-1" when it sees a NULL arg. However, that will cause parse-options to exit(129) without printing anything further, leaving the user confused about what happened. Instead, let's explicitly mark it as PARSE_OPT_NONEG, which will give a useful error message (and print the usual -h output). In theory this could be used to override an earlier "-m", but it's not clear how it would interact with other message options (e.g., would it also clear data read for "-F"?). Since it's already disabled and nobody is asking for it, let's punt on that and just improve the error message. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>