tag: mark "--message" option with NONEG

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>