Savvy investors have been making a meal of Donald Trump’s tendency to climb down from his own threats. - MarketWatch photo illustration/Getty Images, iStockphoto It started as a joke. But nowadays nobody’s laughing. Over the past year, the “TACO trade,” as many on Wall Street call it, has evolved from a humorous acronym, first popularized by a columnist with the Financial Times, into a major force in markets. As crude-oil prices soared in March following the start of the U.S. and Israel’s assault on Iran ...