Inpossible, for multiple reasons
1. Is illegal to do so, since github is for open source, while game is commercial, it is known as piracy also if you do so
2.then steam would destroy everything when you start the game
3.you aren't really expected to fix bugs of commercial game, since it is not open source....
Main thing is that game is not open source and changing code, even touching their files could be considered as illegal, piracy and theft, so impossivle
Maybe in 70 years, iirc, that is when copyright ends, unsure tho, you can make that, but surely not now
Don't even try to do it yourself, you might get heavy fines depending on the country you live in.