[SPARK-2700] [SQL] Hidden files (such as .impala_insert_staging) should be filtered out by sqlContext.parquetFile

I'd prefer to reformat these lines a bit:

```scala
    val children = fs.listStatus(path).filterNot { status =>
      val name = status.getPath.getName
      name(0) == '.' || name == FileOutputCommitter.SUCCEEDED_FILE_NAME
    }
```