Fix up docs where "--" isn't displayed correctly.

A bare "--" doesn't show up in man or html pages correctly
as two individual dashes unless backslashed as \--
in the asciidoc source.  Note, no backslash is needed
inside a literal block.

Signed-off-by: Sean Estabrooks <seanlkml@sympatico.ca>
Signed-off-by: Junio C Hamano <junkio@cox.net>