race: two clients can race and start separate watchers for the same dir
race: two clients can race and start separate watchers for the same dir

Seems to happen rarely, but I have seen the logs that prove it.
The remediation is pretty simple; we just need to check to see if
someone beat us to registering the root that we're creating and
return that one instead.

dhruvsinghal/watchman,michalliu/watchman,michalliu/watchman,facebook/watchman,nodakai/watchman,wez/watchman,dcolascione/laptop-watchman,kwlzn/watchman,wez/watchman,facebook/watchman,kwlzn/watchman,wez/watchman,dhruvsinghal/watchman,michalliu/watchman,wez/watchman,nodakai/watchman,dcolascione/laptop-watchman,facebook/watchman,dcolascione/laptop-watchman,dcolascione/laptop-watchman,dhruvsinghal/watchman,wez/watchman,kwlzn/watchman,nodakai/watchman,dhruvsinghal/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,dhruvsinghal/watchman,michalliu/watchman,nodakai/watchman,michalliu/watchman,nodakai/watchman,dhruvsinghal/watchman,dcolascione/laptop-watchman,facebook/watchman,facebook/watchman,wez/watchman,wez/watchman,michalliu/watchman,kwlzn/watchman,facebook/watchman,kwlzn/watchman,michalliu/watchman,dhruvsinghal/watchman,nodakai/watchman,kwlzn/watchman,dhruvsinghal/watchman,facebook/watchman,wez/watchman,kwlzn/watchman,wez/watchman,facebook/watchman,michalliu/watchman,dcolascione/laptop-watchman,dcolascione/laptop-watchman,dcolascione/laptop-watchman,nodakai/watchman