Monkey patch to preserve GIT_ env variables

Current any cookbook launched with the dev_cookbooks tag is broken if the git chef provider is used at all.

This tag causes cookbooks to be downloaded via the git rubygem. This rubygem unfortunately sets 3 GIT_ environment variables that will screw up any subsequent shell out to git:  https://github.com/schacon/ruby-git/blob/master/lib/git/lib.rb#L655-L657

[Details](https://rightsite.gitsrc.com/trac/ticket/16378)