def is_git_dir(d):
    """ 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."""
    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