Imagine a static web site compiled with jekyll and hosted on github pages. Is it possible to have some javascript on this page that asks the user some input, and then "git push" this input into some github repository ?
In other words, how to write in javascript the following program:
If you know the internals of git (blobs, trees, commits), then you should be able to implement a solution with the GitHub REST API. This is not going to be done in 5 minutes. As for security, you should be able to use OAuth, so no need for private ssh key.
Have a look at: https://developer.github.com/v3/git/