make test-patch plugins more consistent

In response to: https://issues.apache.org/jira/browse/HADOOP-12207?focusedCommentId=14637515&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14637515

• make *_calcdiffs to be a common function
• move add_footer_table after all examinations finished
• replace ${PATCH_BRANCH} in output filenames with "branch-"