KT Tunstall has figured out cause-concert couture. On the bottom, as a pop star, gold-lame pants. On top, for the summer heat and the cameras, a T-shirt with a big slogan about saving something. And sling a guitar covering the bottom half of the shirt so people have to keep watching for a glimpse of what she’s saving: “Save the Future.” At a concert that’s going to be filled with environmental admonitions, she offered, with a smile, “Eat more soil or something.”