Set ghprbTargetBranch parameter when triggering Jenkins.
Set ghprbTargetBranch parameter when triggering Jenkins.

This is necessary for our SQL-change-detection code to work properly;
see https://github.com/apache/spark/pull/6629

databricks/spark-pr-dashboard,mengxr/spark-pr-dashboard,d3borah/spark-pr-dashboard,d3borah/spark-pr-dashboard,databricks/spark-pr-dashboard,mxm/spark-pr-dashboard,mengxr/spark-pr-dashboard,databricks/spark-pr-dashboard,d3borah/spark-pr-dashboard,mxm/spark-pr-dashboard,mxm/spark-pr-dashboard,mengxr/spark-pr-dashboard