Contemporary blockbuster filmmaking is all about the origin story, so naturally, in “Death on the Nile,” his follow-up to the 2017 Agatha Christie mystery “Murder on the Orient Express,” director/star Kenneth Branagh details the nascence of one of his most fascinating recent screen creations: Hercule Poirot’s mustache. In “Murder on the Orient Express,” the seemingly sentient facial hair wasn’t worn by Branagh so much as it wore him, stretching from cheek to cheek in a feathered style of many layers that seemed to move almost independently of each other. In “Death on the Nile” the tragic birth…