function downloadMultifilesTestTarget(cb) {
	if (test("-d", PERF_MULTIFILES_TARGET_DIR)) {
		process.nextTick(cb);
	} else {
		mkdir("-p", PERF_MULTIFILES_TARGET_DIR);
		echo(
			"Downloading the repository of multi-files performance test target.",
		);
		exec(
			`git clone -b v1.10.3 --depth 1 https://github.com/eslint/eslint.git "${PERF_MULTIFILES_TARGET_DIR}"`,
			{ silent: true },
			cb,
		);
	}
}
Downloads a repository which has many js files to test performance with multi files.
Here, it's eslint@1.10.3 (450 files)
@param {Function} cb A callback function.
@returns {void}