However many shares it was, it recently grew 49-fold thanks to a stock split.Based on sales, it's clear plenty of people enjoyed a burrito, salad, or quesadilla at Chipotle Mexican Grill (CMG -3.41%) this year. Billionaire hedge fund manager Bill Ackman is also enjoying the restaurant chain, but not necessarily for its food. It's been a great investment for Ackman's Pershing Square Capital Management and now stands as its top holding.Ackman can buy a lot of tacosPershing Square's latest 13F regulatory filin ...