git config: don't allow multiple config file locations

Either --global, --system, or --file can be used, but not any
combination.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-config.c

@@ -316,6 +316,11 @@ int cmd_config(int argc, const char **argv, const char *unused_prefix)
 	argc = parse_options(argc, argv, builtin_config_options, builtin_config_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 
+	if (use_global_config + use_system_config + !!given_config_file > 1) {
+		error("only one config file at a time.");
+		usage_with_options(builtin_config_usage, builtin_config_options);
+	}
+
 	if (use_global_config) {
 		char *home = getenv("HOME");
 		if (home) {