Emotions turn on a dime in The Porch on Windy Hill, the “new play with old music” at Urban Stages. They’re illogical and inconsistent, and that’s why you’ll probably enjoy the old music more than the new play, which is by Sherry Stregack Lutken, Lisa Helmi Johanson, Morgan Morse, and David M. Lutken. But as for the old music, you are likely to enjoy it very much indeed.