fix: Update queries to work with latest tree-sitter-gitcommit

I found that at least when using revision `aa5c279287f0895a7ebc76a06e55ac3e4b2df7c7` of `tree-sitter-gitcommit`, the `subject_prefix` token is now called `prefix`. Renaming the token allows the font lock to work once again for me. **BREAKING CHANGE:** Instead of querying for `subject_prefix`, we now query for `prefix`. This may have negative consequences for previous versions of `tree-sitter-gitcommit`. FIXES: #5