Fiat Chrysler Automobiles NV and the United Auto Workers union agreed to extend their current four-year contract, set to expire at midnight yesterday, while they keep negotiating toward a new accord, Bloomberg News reported today. The existing agreement will be extended on an hour-by-hour basis as bargaining continues, the union said. The UAW usually reaches a deal with one company first, often the strongest, in hopes of setting a template for pay and benefits for the other two Detroit automakers to follow. This year, the union bucked that tradition, choosing the weakest of the three after Fiat Chrysler laid bare its need for a merger partner.