0d2f10d introduced a regression that broke the arity check in
`set-tree-sitter!` for pre-30 compatibility, because `func-arity`
returned the arity of the advice, not the advised function, so the
`commit` argument was always stored in `treesit-language-source-alist`,
even before it was supported, resulting in wrong-number-of-args errors
for users on Emacs <=29.
Amend: 0d2f10dcab