message: Fix build issues when having packed git refs.
  This is a workaround to fix build failure caused by packed git refs.
  The tf.__git_version__ string will be "unknown" in this case.
diff: @@ -114,6 +114,13 @@ def configure(src_base_path, gen_path, debug=False):
  for target, src in link_map.items():
  if src is None:
  open(os.path.join(gen_path, target), "w").write("")
  + elif not os.path.exists(src):
  + # Git repo is configured in a way we don't support such as having
  + # packed refs. Even though in a git repo, tf.__git_version__ will not
  + # be accurate.
  + # TODO(mikecase): Support grabbing git info when using packed refs.
  + open(os.path.join(gen_path, target), "w").write("")
  + spec["git"] = False
  else:
  try:
  # In python 3.5, symlink function exists even on Windows. But requires
  