Fixed ambiguous ls-remote with refs/heads/...

This is the smallest possible change to solve the
issue of CI builds being triggered again and again.
ls-remote will always be definite when "refs/heads/<branchName>"
is used as branchSpec (except if branch names start with "refs/heads/" which no one should do - but could).

I wrote down my findings and thoughts here:
https://github.com/alexanderlink/git-plugin/tree/branchSpecDiscussion

All other issues tried to solve with Pull Request
https://github.com/jenkinsci/git-client-plugin/pull/129
are still open. But using refs/heads/ would already solve
the CI issue.