Chipotle shares have climbed in the triple digits in recent years.Chipotle Mexican Grill (CMG -0.12%) is doing something it's never done before in its 30-year history. The company is splitting its stock, with the operation scheduled for this week. The decision came after the shares soared in the triple digits over the past few years, reaching beyond $2,000 last year and past $3,000 this year.The reason for such enormous gains? This fast-casual restaurant has reported quarter after quarter of growth -- even ...