Revert "Revert "Use libgit2 for driving git instead of the CLI""

This reverts commit 04440bb035082d4163f2211af447b8d67e077ccb.

Conflicts:
	src/cargo/sources/git/utils.rs
Update git2-rs with mingw support

An upstream PR for libgit2 was applied locally, modified to build, and then
git2-rs was updated to use this small for of libgit2. This adds https support
via winhttp on mingw for windows users.
Roll back git2 a bit to fix tests
auto merge of #446 : alexcrichton/cargo/libgit2-again, r=brson

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.