* Optimism on U.S. budget talks lifts equities, supports oil * Coming up: API oil data 4:30 p.m. EST Tuesday By Robert Gibbons NEW YORK, Dec 17 (Reuters) - Brent crude prices dipped on Monday while U.S. oil futures rose as news of a key U.S. pipeline expansion will be completed next month and optimism about a deal to avoid the "fiscal cliff" prompted spread trading between the two contracts. The discount of U.S. crude to the international Brent benchmark narrowed by more than