[EDIT] Everyone beat me to it! :pinch:
Anders Kaare's twitter reply is probably a better bet than submodules (much internet whining has been done about git submodules):
https://twitter.com/sqaxomonophonen/status/615588052614926336
Say you have
github.com/hmh/handmadehero
So make
github.com/hmh/handmadehero_community but don't push to it yet, and don't tell it to add default files. You want to take a local clone of the master Casey repo and redirect it to the community repo. This won't be an ordained Fork in the Land of Github, in that their database doesn't store a special pointer from one to the other, but it's effectively a fork.
[pre]
git clone git://
github.com/hmh/handmadehero.git
mv handmadehero handmadehero_community
cd handmadehero_community
git remote set-url origin git://
github.com/hmh/handmadehero_community
git remote add origin-casey git://
github.com/hmh/handmadehero
[/pre]
Now someone who has cloned hmh/handmadehero_community can also fetch and merge changes from hmh/handmadehero, and subsequently push those changes to hmh/handmadehero_community if they are on the contributor list for that repo.
[pre]
cd handmadehero_community
git pull origin master
git pull origin-casey master
# clean up the mess...
git push origin master
[/pre]