Before this change, List would return incorrect directory paths (relative to the
wrong root) if the Fs root pointed to a subdirectory. For example, listing dir
"a/b/c/d" of remote :memory: would work correctly, but listing dir "c/d" of
remote :memory:a/b would not, and would result in "Entry doesn't belong in
directory %q (contains subdir)" errors.
This change fixes the issue and adds a test to detect any other backends that
might have the same issue.
f62e7b5b30