[SPARK-11596][SQL] In TreeNode's argString, if a TreeNode is not a child of...

[SPARK-11596][SQL] In TreeNode's argString, if a TreeNode is not a child of the current TreeNode, we should only return the simpleString.

In TreeNode's argString, if a TreeNode is not a child of the current TreeNode, we will only return the simpleString.

I tested the [following case provided by Cristian](https://issues.apache.org/jira/browse/SPARK-11596?focusedCommentId=15019241&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15019241).
```
val c = (1 to 20).foldLeft[Option[DataFrame]] (None) { (curr, idx) =>
    println(s"PROCESSING >>>>>>>>>>> $idx")
    val df = sqlContext.sparkContext.parallelize((0 to 10).zipWithIndex).toDF("A", "B")
    val union = curr.map(_.unionAll(df)).getOrElse(df)
    union.cache()
    Some(union)
  }

c.get.explain(true)
```

Without the change, `c.get.explain(true)` took 100s. With the change, `c.get.explain(true)` took 26ms.

https://issues.apache.org/jira/browse/SPARK-11596

Author: Yin Huai <yhuai@databricks.com>

Closes #10079 from yhuai/SPARK-11596.