> +            (if commit
> +                ;; Force blobless full clone to be able later
> +                ;; to checkout a commit (bug#78542).
> +                (let ((treesit--install-language-grammar-blobless t)
> +                      (treesit--install-language-grammar-full-clone t))
> +                  (treesit--git-clone-repo url revision workdir))
> +              (treesit--git-clone-repo url revision workdir)))

Since with this change it's possible to specify the commit,
let's also improve the format of the source list.
Currently adding a commit to the list requires
prefixing it with four nils:

  (treesit--install-language-grammar-1
   (locate-user-emacs-file "tree-sitter") 'json
   "https://github.com/tree-sitter/tree-sitter-json"
   nil nil nil nil "4d770d3")

The following patch introduces an alternative format
using keywords, e.g.:

  (treesit--install-language-grammar-1
   (locate-user-emacs-file "tree-sitter") 'json
   "https://github.com/tree-sitter/tree-sitter-json"
   :commit "4d770d3")