DefaultBuildChooser.getCandidateRevisions() now returns Set<Revision> to filter duplicates

This pull request depends on [this pull request](https://github.com/jenkinsci/git-client-plugin/pull/154) in git-client-plugin
the same:
https://wiki.jenkins-ci.org/display/JENKINS/Governance+Document#GovernanceDocument-Usingpullrequests