Not to discourage you, but really, given the fact you did not manage to find
contributor's guidelines for Git, Git for Windows (and Chromium you have
referred to in another message to this list, which has nothing to do with that
project), I would say you're not yet on par to sensibly consribute there
because these projects are reasonably hard-core (in different respects, but
this does not matter).
I do understand that you're probably concerned with creating a portfolio which
would supposedly ease job applications for you, and that is okay, but please
consider starting with lower profile F/OSS projects before embarking on top
tier ones. I mean, when you will be ready to embark on such projects, you
won't be asking questions like this one I'm answering ;-)
To make things clear, everything written above is my personal opinion, and,
when it comes to Git, Git for Windows (and Chrome) I'm a mere bystander.
I happen to contribute to Git by trying to help people out on the mailing
lists related to it, but that is all. I mean, the actual developers of Git and
Git for Windows can of course maintain an opinion different from mine.
Still, my opinion is based on being involved in F/OSS for, like, more than 20
years, so I think I do have certain perspective.