Nu is a surprising Buffett stock.Investors trust Warren Buffett for a reason. Although he's the first to say he's made mistakes, he has a proven track record of picking winners and beating the market.One of his more curious picks is Nu Holdings (NU 0.46%). That's not because Nu isn't a great stock; it's up 54% this year, and it's reporting incredible results. It's unusual because it's a growth stock in a risky region, which isn't usually Buffett's style.If Buffett's bullish on this stock, though, it may not ...