Peter Jackson’s new, nearly eight-hour edit of the 1969 film “The Beatles: Get Back” is getting plenty of attention, along with its fair share of rave reviews and withering criticism. The documentary, cleaned up with the latest technology, counters the usual story of the Beatles’s acrimonious breakup by showing them doing more than squabbling. They collaborate, joke around and wax nostalgic in studios and in their legendary rooftop concert. The film reminds us that at the end of the 1960s, they were still writing innovative music that resonates today.