[SPARK-9810] [BUILD] Remove individual commit messages from the squash commit message

For more information, please see the JIRA ticket and the associated dev list discussion.

https://issues.apache.org/jira/browse/SPARK-9810

http://apache-spark-developers-list.1001551.n3.nabble.com/discuss-Removing-individual-commit-messages-from-the-squash-commit-message-td13295.html

Author: Reynold Xin <rxin@databricks.com>

Closes #8091 from rxin/SPARK-9810.

dev/merge_spark_pr.py

@@ -159,11 +159,7 @@ def merge_pr(pr_num, target_ref, title, body, pr_repo_desc):
         merge_message_flags += ["-m", message]
 
     # The string "Closes #%s" string is required for GitHub to correctly close the PR
-    merge_message_flags += [
-        "-m",
-        "Closes #%s from %s and squashes the following commits:" % (pr_num, pr_repo_desc)]
-    for c in commits:
-        merge_message_flags += ["-m", c]
+    merge_message_flags += ["-m", "Closes #%s from %s." % (pr_num, pr_repo_desc)]
 
     run_cmd(['git', 'commit', '--author="%s"' % primary_author] + merge_message_flags)