fix to compareRemoteRevisionWithImpl

    String singleBranch was not being passed to git.getHeadRev(...).
    If the branch was represented by an environment variable, and the check would be done using the 'unexpanded' value from getBranches().get(0).getName().

    The effect of this is that the poller picks up any changes in the repository, not just those for the specific branch, and continously triggers the build unnecessarily.