That was my first thought, varnish (vinyl these days) in front of the website should dramatically reduce this as the git repo should be practically static for most of the content.
But there are "cubic bazillions" of possible URLs that are being requested. Even if they boil down to "only" some millions actual commits, their rendered HTMLs are all different.
Yes, this is very difficult to solve for sites with many URL variations, like git repos and heavily threaded forums. Your cache is always full, but the hit ratio is abysmal.