Tweak the install script for windows nightlies
Rewrite git tests to not rely on the CLI

The CLI has started to have differences on windows than on other systems, and
instead of coding against multiple CLI versions, instead just rewrite everything
to use libgit2.
auto merge of #574 : rust-lang/cargo/fix-win64, r=brson

These commits fix all tests and installation utilities on win64. I've confirmed that these are a green build on the bots.