By Oliver Griffin and Carlos Vargas BOGOTA (Reuters) – Colombians are voting on Sunday to choose a new president from among a former rebel promising generous social programs, a center-right candidate warning against a leftist economic model, and an eccentric business magnate. Gustavo Petro, a leftist former mayor of Bogota and member of the M-19 guerrilla group and current senator, is consistently leading opinion polls with around 40% support, 10 points below what he would need to secure the presidency without a June second round. The 62-year-old has attracted backing on promises to redistribu…