Not that big of a deal, depending on the size of the engine diesels generally have a very large oil
capacity. That being said, it is best to run it with the proper amount of oil, as too much in the crankcase can whip up and foam the oil (although I doubt a quart would do this).
Simple solution, remove the oil filter
, drain the filter, screw it back on, start the engine and check the oil. The filter will hold 1/2 to 1 quart depending on the size of your engine. You may need to do it twice to get the oil level right.