Thanks for taking the time to contribute to Git! Please be advised that the Git community does not use github.com for their contributions. Instead, we use a mailing list (email@example.com) for code submissions, code reviews, and bug reports.
Nevertheless, you can use GitGitGadget to conveniently send your Pull Requests commits to our mailing list.
If you prefer video, then this talk might be useful to you as the presenter walks you through the contribution process by example.
Your friendly Git community!