class Bundler::Source::Git::GitProxy

def git_null(command)

will get the $stderr messages as well.
If it doesn't, everything will work fine, but the user
the best solution is to pipe to /dev/null if it exists.
Given that open3 is not cross platform until Ruby 1.9.3,
TODO: Do not rely on /dev/null. 
def git_null(command)
  git("#{command} 2>#{Bundler::NULL}", false)
end