github uses libgit2 https://github.com/libgit2/libgit2 (in C) but I'm pretty sure they have a lot of wrappers around it (maybe a ruby gem with native extensions, maybe using ffi)

edit: ruby gem: https://github.com/libgit2/rugged which has a native extension written in C to bind libgit2 functions to ruby methods.

They almost surely did not implement Git in Ruby, but merely wrote bindings to it. I used to remember reading through their Ruby source code for this (Edit: found it, it's called Rugged). But even then, you need a more efficient API layer for this binding to be more practical for programmers, hence the libgit2.