KABUL (Reuters) - NATO-led forces in Afghanistan were tricked into helping a group of U.S. vigilantes hunting for militants and who were arrested for illegally detaining Afghans, a spokesman said on Wednesday. Three U.S. nationals were arrested last week after a brief shootout in Kabul. They and some Afghan accomplices had been illegally detaining and interrogating people they believed to be terrorists, Afghan officials said. The group had asked for, and received, help from members of Afghanistan's NATO-led peacekeeping force, who believed they were members of a genuine military task force, a spokesman for the International Security Assistance Force (ISAF)...