The benchmark gauge of the US stock market, the S&P 500, briefly poked its head above water Thursday, as the index moved into positive territory for the first time during 2016. Alas, the blue-chip index couldn’t hold its scant gains through the close of trading. At any rate, the Federal Reserve’s decidedly dovish tone in yesterday’s monetary policy announcement seems to have put a bit of gas in investors’ engines, as has the rebound in some commodity prices. In fact, oil price gains have been outpacing stocks this year.