I still recommend people to watch this video from the man himself, Linus Torvalds, if they really want to understand the architecture and data structure behind git - https://www.youtube.com/watch?v=4XpnKHJAok8

If I remember correctly, he mentioned that he wrote the MVP in 4 weeks and the data structures he used are quite simple. Never got a chance to look at the source code but I guess they are in github (at least the mirror copy.)