This oil giant has lagged its leading rivals through two energy crises. Now one Wall Street giant says it's time to buy.

By Steve GoldsteinBP stock is upgraded to 'overweight' by JPMorganBP stock has lagged its rivals through two energy crises.Integrated oil giant BP has lagged its closest rivals over the last three to five years, but now one leading Wall Street firm says it's time to buy the beleaguered company.JPMo…