[PATCH] Escape asciidoc's built-in em-dash replacement

[PATCH] Escape asciidoc's built-in em-dash replacement

AsciiDoc replace '--' with em-dash (&#8212) 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>