<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_git_tracked_files(rootdir='.'):
 """Parsing .gitignore rules is hard. However, a way we can get around this problem by just listing all currently tracked git files, and start our search from there. After all, if it isn't in the git repo, we're not concerned about it, because secrets aren't being entered in a shared place. :type rootdir: str :param rootdir: root directory of where you want to list files from :rtype: set|None :returns: filepaths to files which git currently tracks (locally) """

    try:
        with open(os.devnull, 'w') as fnull:
            git_files = subprocess.check_output(
                [
                    'git',
                    'ls-files',
                    rootdir,
                ],
                stderr=fnull,
            )

        return set(git_files.decode('utf-8').split())
    except subprocess.CalledProcessError:
        return None