Auto merge of #4602 - ehuss:fix-new-test-windows, r=alexcrichton

Allow test to pass if user has ~/.gitconfig on Windows.

The `new` test will fail on Windows if you have `~/.gitconfig` configured with a username/email.  This is because libgit2 searches quite a few environment variables hunting for a config file.  Rather than adjusting other environment variables (I think at a minimum it would be `%HOMEDRIVE%%HOMEPATH%` and `%USERPROFILE%`), this just adds an empty config file since libgit2 will stop at the first one it finds.