Banksy, the British street artist whose paintings have fetched as much as $1.9 million at auction, keeps his identity secret to avoid prosecution for graffiti. He introduced his debut film at the Berlin Film Festival via video message in a hooded sweater, his voice disguised. “Exit Through the Gift Shop” is his “film about a man who tried to make a film about me,” Banksy says. The story he tells is hilariously outrageous....