Gollum is \"A simple, Git-powered wiki with a sweet API and local frontend.\"
It\'s hosted on GitHub: http://github.com/github/gollum
It seems to be a simple Si
As already mentioned, the problem is that the heroku filesystem is readonly. But the real problem is underlying grit, which relies on the git command line tool. You can't work with remote repositories without cloning them to the local directory.
See the related question.
So, the solution will be to clone the repo to temporary path, work there and push changes to the remote repo. There is a much overhead: you need to clone repo every time a user browse a wiki page.
Another solution that comes to mind is making some API for grit that will enable to work with git remotely.
Yet another solution is to work with git over ssh.