Shares of Chipotle Mexican Grill tumbled as much as 19% in morning trading on Thursday after the company cut its full-year same-store sales forecast for the third straight quarter.Including Thursday's move, the stock has fallen 45% this year, dragging its market value down to roughly $43 billion. At least five Wall Street analysts have cut their price targets for the stock after the report, anticipating investors' displeasure with the burrito chain's shrinking traffic and gloomy outlook."It's difficult to c ...