module Sprockets::PathUtils

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

subpath - String subpath of path
path - String path

Internal: Get relative path for root path and subpath.

def split_subpath(path, subpath)
  return "" if path == subpath
  path = File.join(path, '')
  if subpath.start_with?(path)
    subpath[path.length..-1]
  else
    nil
  end
end