By Luis Jaime Acosta and Nelson Bocanegra BOGOTA (Reuters) -Gustavo Petro will on Sunday become Colombia’s first leftist president, elected by voters who hope he can carry out ambitious social and economic reforms meant to reduce violence and deep inequality in the polarized Andean country. Petro, a former member of the M-19 guerrillas, is set to be inaugurated in Bogota’s Bolivar Plaza on Sunday afternoon. Thousands of supporters were gathering in central Bogota and at large screens set up in public places around the country. “I didn’t believe I would live to see this finally happen,” said Ne…