code: def is_git_dir(d):
      
      if osp.isdir(d):
          if osp.isdir(osp.join(d, 'objects')) and osp.isdir(osp.join(d, 'refs')):
              headref = osp.join(d, 'HEAD')
              return osp.isfile(headref) or \
                  (osp.islink(headref) and
                   os.readlink(headref).startswith('refs'))
          elif (osp.isfile(osp.join(d, 'gitdir')) and
                osp.isfile(osp.join(d, 'commondir')) and
                osp.isfile(osp.join(d, 'gitfile'))):
              raise WorkTreeRepositoryUnsupported(d)
      return False
docs: This is taken from the git setup.c:is_git_directory
      function.
  
      @throws WorkTreeRepositoryUnsupported if it sees a worktree directory. It's quite hacky to do that here,
              but at least clearly indicates that we don't support it.
              There is the unlikely danger to throw if we see directories which just look like a worktree dir,
              but are none.