When the old curmudgeon Edgar Degas died in 1917, a stunning trove of works by Edouard Manet — eight paintings, 14 drawings and 60 prints — was discovered in his studio. There, too, was a portrait of Manet and his wife Suzanne, painted by Degas 50 years earlier. But its right-hand third was missing — which included half of Suzanne’s body and all of the piano she was playing.