In 1996, the Libyan writer Hisham Matar was living near the National Gallery in London. For six years straight he had been going to the museum, sometimes as often as five times a week, to look at the same painting by Velázquez. But on June 29 Matar left off with the Velázquez and switched to a new painting, Manet’s The Execution of Maximilian. Years later, he learned that the day he took up his “vigil” before Manet’s depiction of a death by firing squad was, in all likelihood, the same day that his father was gunned down alongside more than…