message: copy: Add assertion.
diff: @@ -3187,6 +3187,7 @@ def __copy_stripped(source, to, strip_prefix, builder, post_process, follow_syml
  __copy_stripped_cache[key] = res
  return res
  res = builder(source, path, post_process, follow_symlinks).target
  + assert res is not source
  for dep in source.dependencies:
  if not dep.name_absolute().absolute():
  node = __copy_stripped(dep, to, strip_prefix, builder, post_process, follow_symlinks)
  