Revert the revert of libgit2

I applied https://github.com/libgit2/libgit2/pull/2414 to a [local branch](https://github.com/alexcrichton/libgit2/tree/mingw-w64-winhttp) and then fixed the build for mingw-w64. The tests pass fine on mingw and I've used https successfully on windows with libgit2 on cargo now.