iothread: make sure that paths are descendant of the root
iothread: make sure that paths are descendant of the root

Summary:
In some rare situation, when a directory creation races with adding a watch on
that directory, it looks like FSEvent may be sending us notification about the
parent directory being watched.

Reviewed By: chadaustin

Differential Revision: D27083891

fbshipit-source-id: 7210550f3ae43a123e7eda06f96f5212ee5e402a

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