I disagree that you need to know the internals. You need to understand an abstract model, that may have been influenced by internals of early versions, and part of such internals may have been preserved into current versions. But unless you want to modify git, you should not have to know anything about the internals. The abstract model is enough. I consider to know virtually nothing of git internals, yet I consider to be proficient in understanding its abstract model and using it.