prevent watchman from crashing if a path within the project is requested
prevent watchman from crashing if a path within the project is requested

Summary: Watchman would crash if the path requested to be watched is under the repository root. This diff fixes that.

Reviewed By: xavierd

Differential Revision: D37672768

fbshipit-source-id: a50d1023fac179089476b4260a86a0d0ea1e0701

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