Fixed help and added docu for namespaced branches

See git-client-plugin Pull Request
https://github.com/jenkinsci/git-client-plugin/pull/120
Reverted wrong change regarding "any branch"
Switched to new git-client 1.6.4 containing namespace fix
Merge pull request #207 from alexanderlink/namespace-branch-help

Fixed help and added docu for namespaced branches