[JENKINS-25632]

Start off by empty repository instead of having an empty commit.
This way, people can git-push existing workflow libraries into
a new Jenkins instance without using the forced push.

While writing a test, I also discovered a bad interaction with CSRF protection
and git-server plugin, which I fixed in git-server plugin 1.5.
[JENKINS-25632]

form validation on the CPS script should honor the actual Groovy compiler setting to be used,
including global shared lib and auto imports.