SAN FRANCISCO — Trevor McDonald flew from Salt Lake City to Philadelphia last week, watched two horrible losses, and then flew right back. He was the 27th man for a doubleheader in Philadelphia, but he didn’t pitch, as the Giants elected to stick with Adrian Houser even though that would lead to the need for a spot start on Monday. It was a long couple of days, but McDonald bounced back quickly. When he landed back in Salt Lake, he did what you might expect from a 25-year-old. “I slept for about 14 hours,” he explained Monday night.That marathon session got him back to form, and for McDonald, that usually means just one thing when he’s in the big leagues. He has made cameos in each of the past three seasons, and he tends to dominate. The latest start was seven strong against the San Diego Padres, leading the Giants to a 3-2 win that snapped a six-game skid. It also led to some fascinating questions.The theme of the day Monday was meritocracy. Casey Schmitt is part of an infield that includes three of the biggest contracts in franchise history, but he’s the one who has a firm grip on a job right now because of how...