One man. One white shirt. A column of tanks. And nobody knows who he was. June 5th, 1989, the morning after the Chinese government massacred its own people in Tiananmen Square.   A single man stepped in front of a column of tanks and refused to move. He blocked them every time they tried to go around him, climbed on top of the lead tank, and spoke to the crew inside. Then he vanished.   A British newspaper said his name was Wang Weilin. Some say he was imprisoned. Others say executed. Chinese of