If you are the author (and every other contributor signed relevant CLAs) then you can distribute the code however you want. You do not have to include game assets in the code distribution.
I do realize that. However, in the case of 0ad, they do not require such a CLA. Rather their website has this to say:
> By sending in a patch, you agree that it is your own work (or else make it clear where it came from) and agree to licensing any code as GPL 2+ (or in some cases MIT, especially for code in lib/ - check the existing copyright headers on the source files you edit) and data files as CC-BY-SA 3.0.
