“Once upon a time, revolutionary opera was forced down the Chinese people’s throats.” One such work, “Jiang Jie,” is on the program of the opera festival at Beijing’s National Center for the Performing Arts. It would be easy to assume that its presence “is an obligation imposed by the government. Except that this festival sells tickets on the free market. Those tickets aren’t cheap. And the audience for ‘Jiang Jie’ on Friday night is large, and notably involved.”