[checkstyle-tester] minor: diff.groovy refactoring
[checkstyle-tester] minor: diff.groovy refactoring

MEZk/contribution,checkstyle/contribution,rnveach/contribution,Vladlis/contribution,MEZk/contribution,checkstyle/contribution,rnveach/contribution,Vladlis/contribution,Vladlis/contribution,checkstyle/contribution,MEZk/contribution,rnveach/contribution,rnveach/contribution,Vladlis/contribution,checkstyle/contribution,MEZk/contribution,Vladlis/contribution,checkstyle/contribution,checkstyle/contribution,rnveach/contribution,MEZk/contribution,checkstyle/contribution,Vladlis/contribution,rnveach/contribution,MEZk/contribution,rnveach/contribution