* TSX up 138.77 points, or 1.1 percent, at 12,454.89 * Materials sector leads index up amid U.S. budget uncertainty * Canpotex deal boosts potash companies By Allison Martell TORONTO, Dec 31 (Reuters) - Canada's main stock index rose more than 1 percent on Monday, boosted by the materials sector, as U.S. lawmakers pushed for a last-minute agreement to avoid the "fiscal cliff" that could put the U.S. economy into recession. Gold rallied in the afternoon after news of a poten