You're granting GitHub the right to display your content to other users, and to make additional copies of it ("forks"). Those seem like reasonable rights to assign to Github when agreeing to their terms of service. I think it's completely clear that putting "all rights reserved" doesn't allow you to opt out of certain parts of the ToS, any more than the "I do not allow Facebook to use my content..." pseudo-legalese borderline-sovereign-citizen nonsense that goes viral every time Facebook change their privacy policy.