Include jenkins special branch name to make it work.

- Jenkins uses git checkout <sha1> in order to clone a branch,
  instead of git clone. This leads to the problem, that the
  checked out repository is not on a branch. A check for branch master
  will fail.
  Jenkins is now configured to create a local branch after git checkout
  which is called jenkins-master, and jenkins-master is included
  as a allowed master branch name.