Make sure that Windows gets the correct Git parameters.

This needs a test before landing. Anyone got any good ideas? We currently have no coverage around `lib/cache/add-remote-git.js` in the tap tests, so my idea of snowcloning an existing test and adding `process.platform = "win32"` at the beginning (which, sadly, works) isn't going to work.

I don't really like the idea of making this test depend on GitHub's availability, but I don't know how to run up a git server very fast, either.