Retrieve one option from Git config.

def load_val_from_git_cfg(cfg_key_suffix):
    """Retrieve one option from Git config."""
    cfg_key = f'cherry-picker.{cfg_key_suffix.replace("_", "-")}'
    cmd = "git", "config", "--local", "--get", cfg_key
    try:
        return (
            subprocess.check_output(cmd, stderr=subprocess.DEVNULL)
            .strip()
            .decode("utf-8")
        )
    except subprocess.CalledProcessError:
        return None