fix(args): Correct legacyArgs logical errors

Close spf13#1156
Signed-off-by: Dong Gang <dong.gang@daocloud.io>

args.go

@@ -9,16 +9,15 @@ type PositionalArgs func(cmd *Command, args []string) error
 
 // Legacy arg validation has the following behaviour:
 // - root commands with no subcommands can take arbitrary arguments
-// - root commands with subcommands will do subcommand validity checking
-// - subcommands will always accept arbitrary arguments
+// - commands with subcommands cannot take any arguments
 func legacyArgs(cmd *Command, args []string) error {
 	// no subcommand, always take args
 	if !cmd.HasSubCommands() {
 		return nil
 	}
 
-	// root command with subcommands, do subcommand checking.
-	if !cmd.HasParent() && len(args) > 0 {
+	// root command with subcommands, the args should be empty.
+	if cmd.HasSubCommands() && len(args) > 0 {
 		return fmt.Errorf("unknown command %q for %q%s", args[0], cmd.CommandPath(), cmd.findSuggestions(args[0]))
 	}
 	return nil