LISBON—The agent manning the passport control line warned me not to be fooled by the mercifully short line that greeted me upon arrival at Portugal’s Lisbon International Airport on the last Friday in July. She urged me to arrive at the airport three or four hours before my return flight and head straight for border control. “Don’t shop. Come right here.” The front-desk agents at my airport hotel, hearing too many horror stories from guests stuck in hourslong passport lines this summer, similarly suggested a god-awful-early alarm. It was all for naught. There was no line when I got to…