I've been using Neovim for a few days with no problem, so I decided to switch my git config to use Neovim instead of vim. Here's what I put in my global .gitconfig:

[core]
    editor = nvim

And here's the warning when I run git commit now:

Warning: Cannot find word list "en.utf-8.spl" or "en.ascii.spl"
Press ENTER or type command to continue

After pressing a key, it works fine, except that it doesn't do spell checking like vim used to do. Here's what I've got in my .nvimrc to turn on spell checking in git commit messages.

autocmd Filetype gitcommit setlocal spell textwidth=72