With those words, Dave Grohl, Krist Novoselic and Pat Smear came onstage for their first public performance together since Nirvana’s final concert more than 18 years ago. McCartney found himself in the extremely unlikely role of substituting for Kurt Cobain, but they didn’t do a Nirvana song. Instead, they did a shockingly great new tune called “Cut Me Some Slack” that’s certain to get a lot of love on YouTube in the coming days.