Joe and Anthony Russo’s sprawling new MCU film takes a decidedly small-scale approach to the franchise’s favorite post-movie treat. “Avengers: Infinity War” (Spoilers ahead for both “ Avengers: Infinity War ” and its post-credits scenes.) Audiences are going to have to sit tight after the conclusion of Joe and Anthony Russo’s sprawling, ambitious “Avengers: Infinity War” (total running time: 149 minutes) to see the film’s last scene, which plays out at the end of some very long credits (it takes a lot of people to make a multi-million dollar film, after all). This recurring feature is different from other recent Marvel movies because it’s the only one. While there’s never been much rhyme or reason to how many post-credits scenes a Marvel Cinematic Universe can pack in (after “Guardians of the Galaxy, Vol. 2” director James Gunn unspooled not just one, not two, but a full five post-credits scenes), recent films have stuck to two. The Russos only have one, but it’s unmissable. It’s also a scene that ties directly into two upcoming Marvel films, and opens up the history of a beloved superhero characters that Marvel fans are finally set to meet on the big screen next year. […]