don't follow mount points when applying shipit transformation
don't follow mount points when applying shipit transformation

Summary:
If a getdeps project creates a bind mount in a subdirectory, we don't
want the shipit transformer to copy all of the bind mount's contents
to the ship-transformed output.

This manifested when I was using Vagrant inside of Watchman and
`getdeps.py fetch watchman` would copy all of the Vagrant VMs too.

Reviewed By: ahornby

Differential Revision: D33855553

fbshipit-source-id: e2bc50ecfe0d067490c95be05d074d6a844f30d5

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