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
I know _what_ it does, just not _why_, since the added test methods could just as easily be added directly to the existing top-level class.