When DJ mash-ups appeared a few years back, they were all about linking strange bedfellows -- Nirvana and Destiny's Child, the Strokes and Christina. The best ones made you hear something new in both songs involved -- which Collision Course does only in snatches. Billed as the first commercially released mash-up record, this six-song EP is well-constructed: The mix of brassy rhymes and heavy electro-rock on "Big Pimpin'/Papercut" and "Dirt...