fast-export: print usage when no options specified

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fast-export.c

@@ -497,6 +497,9 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	if (argc == 1)
+		usage_with_options (fast_export_usage, options);
+
 	/* we handle encodings */
 	git_config(git_default_config, NULL);