Git vc-git.el versus Emacs vc-git.el versus Gits git.el

Hi,

I have some questions concerning the Emacs Git integration:

 1. What is the difference between git.el and vc-git.el in the
    git/contrib/emacs directory?

 2. There is a vc-git.el in the Emacs top-of-trunk sources, and
    another one in the git/contrib/emacs directory in the Git
    sources. Which one should I use?  If I should use the one in
    Emacs, then why is there one in the Git source tree?

Thanks,
Brent