Defense Secretary Chuck Hagel said Tuesday that he has ordered a 20 percent cut in the number of top brass and senior civilians at the Pentagon by 2019, the latest attempt to shrink the military bureaucracy after years of heady growth. Hagel’s directive could force the Pentagon and military command staffs to shed an estimated 3,000 to 5,000 jobs. That’s a tiny percentage of the Defense Department’s 2.1 million active-duty troops and civilian employees, but analysts said it would be a symbolically important trimming of the upper branches of the bureaucracy, which has proved to be resistant to past