Percent forms are allowed to escape from one context to another

[rgrinberg]

          I added a test - It doesn't go through.

By the way, there's still a way to do something illegal here. One can write:

%{cma:../another-context/foo}

And use the relative path to escape to another context. We should definitely forbid that as well. It's a much larger change though.

Originally posted by @rgrinberg in #10169 (comment)