History, as it is unfolding disturbingly before our eyes these days, has changed the way we view the 1990 John Weidman/Stephen Sondheim collaboration,Assassins, so that members of the audience are likely to respond emphatically as they make connections between the show and today's headlines. Certainly, this was the case at last night's opening of the mostly thrilling, occasionally misfired Encores! Off-Center production at City Center, where laughter and applause stopped the performance about fifteen minutes in. . . .