parseopt: make usage optional

Allow usagestr to be NULL and don't display any help screen in
this case.  This is useful to implement incremental parsers.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.c

@@ -364,6 +364,9 @@ int parse_options(int argc, const char **argv, const struct option *options,
 int usage_with_options_internal(const char * const *usagestr,
 				const struct option *opts, int full)
 {
+	if (!usagestr)
+		return PARSE_OPT_HELP;
+
 	fprintf(stderr, "usage: %s\n", *usagestr++);
 	while (*usagestr && **usagestr)
 		fprintf(stderr, "   or: %s\n", *usagestr++);