Neil Diamond, Mary J. Blige, Sugarland, Rob Thomas and Usher are among the many stars performing on Sunday night's holiday special Christmas In Washington, airing Sunday night on TNT at 8 p.m. ET/PT.   The show was taped last weekend in front of an audience that included President and Mrs. Obama. Jennifer Nettles and Christian Bush (JENNIFER) I THINK IT'S A BIG DEAL ON A NUMBER OF LEVELS. ONE, IT'S A BIG DEAL TO BE SINGING FOR THE PRESIDENT OF THE UNITED STATES. (KRISTIAN) THAT'S A GIGANTIC MOMENT. FIRST OF ALL. (JENNIFER) YEAH, THE PRESIDENT OF THE FREE WORLD. "HI! MERRY CHRISTMAS."