If you wish to contribute you can use either the sources located on ryuslash.org or the github sources.
If you wish to work on git-auto-commit-mode you can get the sources by cloning the repository:
git clone git://ryuslash.org/emacs/git-auto-commit-mode.org
Use spaces, don’t use tabs. If you can, keep lines shorter than 80 characters. Other than that, Emacs can handle indentation pretty well.
If you want to send any patches I prefer an email with the output
generated by git request-pull, possibly with -p, over things like
github pull requests.