Fixing JENKINS-24802

Corrected issue explained in https://issues.jenkins-ci.org/browse/JENKINS-24802
Refactored SubversionRepositoryStatus to make onNotify method shorter and more "understandable"
Added unit tests for one refactored method
What is the purpose of this? Just add your new `@Test` methods to the class. (The `setUp` method is overkill anyway; you can just instantiate this class directly at point of use.)