code: def parse_branch_ref(filename):
      """Given a filename of a .git/HEAD file return ref path.
  
      In particular, if git is in detached head state, this will
      return None. If git is in attached head, it will return
      the branch reference. E.g. if on 'master', the HEAD will
      contain 'ref: refs/heads/master' so 'refs/heads/master'
      will be returned.
  
      Example: parse_branch_ref(".git/HEAD")
      Args:
        filename: file to treat as a git HEAD file
      Returns:
        None if detached head, otherwise ref subpath
      Raises:
        RuntimeError: if the HEAD file is unparseable.
      """
  
      data = open(filename).read().strip()
      items = data.split(" ")
      if len(items) == 1:
          return None
      elif len(items) == 2 and items[0] == "ref:":
          return items[1].strip()
      else:
          raise RuntimeError("Git directory has unparseable HEAD")
  
file_path: tensorflow-upstream/tensorflow/tools/git/gen_git_source.py
imports: import argparse
  from builtins import bytes
  import json
  import os
  import shutil
  import subprocess
label_desc: Use image node_id 1 for calling a global function with example usage: parse_branch_ref(filename) and returns: None, items