make query and subscribe work with subdirectory of watched root

Passing the absolute path of a subdirectory to `query` or `subscribe` (in place of a watched root) should work seamlessly.

Here's what would happen. When validating whether the path is a watched root, look at each ancestor until a watched root is found. If the path is not watched but an ancestor is, convert the given path to a `relative_root`. If a `relative_root` was also passed, prepend the converted path to the front of the existing `relative_root`.

This would let clients avoid calling `watch-list` just to find out which ancestor is the valid root. That can be done on Watchman's end.