parse-options: add PARSE_OPT_LITERAL_ARGHELP for complicated argh's

parse-options: add PARSE_OPT_LITERAL_ARGHELP for complicated argh's

Usually, the argh element in struct option points at a placeholder value
(e.g. "val"), and is shown in the usage message as

    --option=<val>

by enclosing the string inside of angle brackets.

When the option is more complex (e.g. optional arguments separated by a
comma), you would want to produce a usage message that looks like

    --option=<val1>[,<val2>]

In such a case, the caller can pass a string to argh with placeholders
already enclosed in necessary angle brackets (e.g.  "<val1>[,<val2>]")
and set this flag.

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