By Nelson Bocanegra, Oliver Griffin and Carlos Vargas BOGOTA/BUCARAMANGA (Reuters) – Colombians were voting on Sunday to elect their next president, choosing between a leftist former guerrilla who is pushing profound social change and an eccentric construction magnate who has promised to fight corruption. Candidates Gustavo Petro, once a member of the M-19 rebels, and Rodolfo Hernandez, who first gathered support through TikTok videos, are technically tied in polling. Petro, a former mayor of capital Bogota and current senator, has pledged to fight inequality with free university education, pe…