“Hair by Sam McKnight” is a retrospective centered on the respected British stylist, who has been a fixture on fashion shoots and the show circuit for 40 years. He was the man who gave Agyness Deyn her crop; slicked back Princess Diana’s hair; styled Madonna’s platinum curls for the cover of Bedtime Stories; and created a flame-red mop that transformed Tilda Swinton into David Bowie. He’s worked with most of fashion’s big names, decade after decade — including, most consistently, Kate Moss, Patrick Demarchelier, and Karl Lagerfeld.