Long answer: Some GNU projects require copyright assignment to the FSF. The most well known example would be GNU Emacs. However, unlike GitHub or another for-profit business, the FSF is a non-profit charity dedicated to free software. Additionally, there is a clause in the CLA to ensure that contributions cannot be made nonfree. I have made some small contributions to a handful of GNU projects at this point and I have not yet had to assign copyright for any of them.
The copyright assignment situation with the FSF and the GNU project is dramatically different than the potential situation that I've described about GitHub. I hope I've made things clear.
