By Laila Kearney NEW YORK (Reuters) – Oil prices whipsawed on Monday, diving early to their lowest since early January but then rebounding as reports varied about whether Saudi Arabia and other OPEC oil producers are considering a half-million barrel daily output increase. Brent crude futures for January fell 77 cents, or 0.9%, to $86.85 a barrel by 12:54 p.m. EST (1754 GMT). U.S. West Texas Intermediate (WTI) crude futures for December were down 58 cents, or 0.7%, at $79.50 ahead of the contract’s expiry later on Monday. Both benchmarks had plunged by more than $5 a barrel early, after the Wa…