parse-options: never suppress arghelp if LITERAL_ARGHELP is set

parse-options: never suppress arghelp if LITERAL_ARGHELP is set

The PARSE_OPT_LITERAL_ARGHELP flag allows a program to override the
standard "<argument> for mandatory, [argument] for optional" markup in
its help message.  Extend it to override the usual "no text for
disallowed", too (for the PARSE_OPT_NOARG | PARSE_OPT_LITERAL_ARGHELP
case, which was previously meaningless), to be more intuitive.

The motivation is to allow update-index to correctly advertise

	--cacheinfo <mode> <object> <path>
	                      add the specified entry to the index

while abusing PARSE_OPT_NOARG to disallow the "sticked form"

	--cacheinfo=<mode> <object> <path>

Noticed-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>