By Ranga Sirilal COLOMBO (Reuters) – Sri Lankan President Mahinda Rajapaksa on Thursday called a presidential election, seeking an unprecedented third six-year tenure, but demands for his power to be curbed and a possible split in his vote base means re-election won’t be a walkover. Rajapaksa, 69, came to power in 2005 and won a second term in 2010 on a wave of popularity after the military defeated Tamil Tiger separatists, ending a 26-year civil war. “I am declaring a secret today. I have signed the proclamation calling for the election for re-election for the third time… …