HAVANA — Over four years of peace talks with the Colombian government, the country’s FARC rebels bargained for the best deal possible — and one that could convince their troops that the past half-century of guerrilla warfare wasn’t for nothing. But in wresting significant concessions from the government, including a launchpad into politics and a chance to skirt prison, the rebels have run the risk of sweetening their deal so much that Colombians won’t stomach it.