Fix for pull request 132


Pull request 132 [1] incorrectly set the proxy in the instance wide EnvVars instead of the method copy.  This caused an issue when multiple repositories are setup with one outside and on inside of the proxy.  This patch fixes the issue by cloning the current env and adding the http_proxy / https_proxy variables to the clone. 

[1] https://github.com/jenkinsci/git-client-plugin/pull/132