teach watchman to look at last matching mount instead of first
teach watchman to look at last matching mount instead of first

Differential Revision: D36389928

fbshipit-source-id: 4775e5612f64e1888a49d67c4185f12df5646c95

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