def split_subpath(path, subpath)
      return "" if path == subpath
      path = File.join(path, ''.freeze)
      if subpath.start_with?(path)
        subpath[path.length..-1]
      else
        nil
      end
    end
def split_subpath(path, subpath)
      return "" if path == subpath
      path = File.join(path, ''.freeze)
      if subpath.start_with?(path)
        subpath[path.length..-1]
      else
        nil
      end
    end
Internal: Get relative path for root path and subpath.

 path    - String path
 subpath - String subpath of path

 Returns relative String path if subpath is a subpath of path, or nil if
 subpath is outside of path.