ON THE TOWN: As predicted last week, Prince made a surprise guest appearance last weekend at a pair of Sheila E. concerts at the Universal Amphitheatre, showcasing the newly enlarged Revolution with a top-funk 20-minute encore. To say the Purple One stole the show would be an understatement. He dove into the audience, performed "Kiss" (and, on Saturday night, "Head") and mercilessly lampooned old pal Morris Day's "Oak Tree" dance hit. Accompanied by his combination backup singer/bodyguards, Prince mimicked Day with a series of mischievous, chopping arm-motions and spastic, stiff-legged dance steps. Prince also appeared at Sheila E.'s Oakland show last Sunday, getting in another dig at Day by boasting, "We're going to chop that oak tree down and turn it into a wooden leg."