As world powers back a cease-fire in Libya to end months of fighting, a recent injection of mercenaries by rival foreign backers threatens to unravel efforts to establish a long-term truce in the oil-rich North African country. At a summit in Berlin on Sunday, Russia, Turkey, France, Italy, Egypt and the United Arab Emirates agreed to support a cease-fire and uphold a previously existing arms embargo in an attempt to quell the fighting. The eruption of a full-blown civil war last year in a country less than 200 miles from…