[SPARK-20616] RuleExecutor logDebug of batch results should show diff to start of batch

## What changes were proposed in this pull request?

Due to a likely typo, the logDebug msg printing the diff of query plans shows a diff to the initial plan, not diff to the start of batch.

## How was this patch tested?

Now the debug message prints the diff between start and end of batch.

Author: Juliusz Sompolski <julek@databricks.com>

Closes #17875 from juliuszsompolski/SPARK-20616.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/rules/RuleExecutor.scala

@@ -122,7 +122,7 @@ abstract class RuleExecutor[TreeType <: TreeNode[_]] extends Logging {
         logDebug(
           s"""
           |=== Result of Batch ${batch.name} ===
-          |${sideBySide(plan.treeString, curPlan.treeString).mkString("\n")}
+          |${sideBySide(batchStartPlan.treeString, curPlan.treeString).mkString("\n")}
         """.stripMargin)
       } else {
         logTrace(s"Batch ${batch.name} has no effect.")