parse-options: allow git commands to invent new option types

parse-options provides a variety of option behaviors, including
OPTION_CALLBACK, which should take care of just about any sane
behavior.  All supported behaviors obey the following constraint:

 A --foo option can only accept (and base its behavior on)
 one argument, which would be the following command-line
 argument in the "unsticked" form.

Alas, some existing git commands have options that do not obey that
constraint.  For example, update-index --cacheinfo takes three
arguments, and update-index --resolve takes all later parameters as
arguments.

Introduces an OPTION_LOWLEVEL_CALLBACK backdoor to parse-options so
such option types can be supported without tempting inventors of other
commands through mention in the public API.  Commands can set the
callback field to a function accepting three arguments: the option
parsing context, the option itself, and a flag indicating whether the
the option was negated.  When the option is encountered, that function
is called to take over from get_value().  The return value should be
zero for success, -1 for usage errors.

Thanks to Stephen Boyd for API guidance.

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

parse-options.c

@@ -66,6 +66,9 @@ static int get_value(struct parse_opt_ctx_t *p,
 		return opterror(opt, "takes no value", flags);
 
 	switch (opt->type) {
+	case OPTION_LOWLEVEL_CALLBACK:
+		return (*(parse_opt_ll_cb *)opt->callback)(p, opt, unset);
+
 	case OPTION_BIT:
 		if (unset)
 			*(int *)opt->value &= ~opt->defval;