checkIndex
This runs the CheckIndex tool on the index in. If any issues are hit, a RuntimeException is thrown; else, true is returned.
fun checkIndex(dir: Directory, level: Int, failFast: Boolean, concurrent: Boolean, output: ByteArrayOutputStream?): CheckIndex.Status
If failFast is true, then throw the first exception when index corruption is hit, instead of moving on to other fields/segments to look for any other corruption.