AsciiDoc replace '--' with em-dash (—) by default. em-dash
looks a lot like a single long dash and it's very confusing when
we are talking about command options.
Section 21.2.8 'Replacements' of AsciiDoc's User Guide says that a
backslash in front of double dash prevent the replacement. This
patch does just that.
Signed-off-by: Yasushi SHOJI <yashi@atmark-techno.com> Signed-off-by: Junio C Hamano <junkio@cox.net>