git plugin 2.0 does not correctly migrate 1.5 configs using reference repo option

The GitSCMBackwardCompatability class does not migrate the reference repo option.

• https://github.com/jenkinsci/git-plugin/blob/master/src/main/java/hudson/plugins/git/GitSCMBackwardCompatibility.java

Note there is no part of readBackExtensionsFromLegacy that adds a CloneOption instance or set the "reference" member.