audit: preallocate watchman_state
audit: preallocate watchman_state

Summary:
HashMap::with_capacity saves some seconds on the final loop, and we
know the size.

Reviewed By: fanzeyi

Differential Revision: D27809457

fbshipit-source-id: 1218b97b0a2ed3a033761b4032e873a6fe242927

nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman