Thanks for taking the time to contribute to Git!
Those seeking to contribute to the Git for Windows fork should see http://gitforwindows.org/#contribute on how to contribute Windows specific enhancements.
If your contribution is for the core Git functions and documentation please be aware that the Git community does not use the github.com issues or pull request mechanism for their contributions.
Instead, we use the Git mailing list (git@vger.kernel.org) for code and documenatation submissions, code reviews, and bug reports. The mailing list is plain text only (anything with HTML is sent directly to the spam folder).
Nevertheless, you can use GitGitGadget (https://gitgitgadget.github.io/) to conveniently send your Pull Requests commits to our mailing list.
Please read the "guidelines for contributing" linked above!