Add an option to mount a checkout read-only
Add an option to mount a checkout read-only

Summary: Added option to mount command

Reviewed By: chadaustin

Differential Revision: D20256352

fbshipit-source-id: e52fbd10f318221777b7e2d21ed9ba0a17ce1687

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