It is usually better to have positive options, to avoid confusing double
negations. However, sometimes it is desirable to show the negative option
in the help.
Introduce the flag PARSE_OPT_NEGHELP to do that.
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>