GitHub is a decent company that offers free blog hosting with the benefits of SSL. Everything is rainbows and sunshine, until you encounter a strange behaviour from Disqus, a popular commenting system that is often used together with GitHub pages, because they only support static sites. GitHub pages do not support automatic redirect to HTTPS (also known as HTTPS only website) and Disqus treats protocol as part of the website URL which is used to load comments. See where this is going? Basically you end up with different groups of comments - those who comment on HTTP and and those who comment on HTTPS. And you are almost stuck with this.
If you just want a quick code snippet that does the redirect, here is one I found in this blog post: GitHub Pages Now Supports HTTPS, So Use It
No matter what I still think that this is an issue Disqus should resolve. I will try to contact them and see if there is anything they can do about it.
Update: Disqus staff explained the situation and provided a better solution for the problem. You just need to specify disqus_identifier in your Disqus embed code. The identifier must be unique for each blog post.