[rst]

Particularly since a git repo, as a whole, isn't a functional data structure. The commits (and the graph in which they're embedded) are immutable, but the mapping between branch names and commits gets mutated all the time. (To say nothing of slightly deeper esoterica like the index and stashes.)

[masklinn]

That's pretty functional still, the "branches" are just (in clojure terms) atoms, aka atomically mutable references to a structure (namely a commit which is some metadata + a reference to a tree).