Enable Copilot to generate Git commit messages based on staged changes

I've configured `copilot.vim` to work on Git commit message as follows:

```
let g:copilot_filetypes = {
\ 'gitcommit': v:true,
\}
```

However, the suggestions don't seem to take into account the staged changes.

Related to https://github.com/github/copilot.vim/issues/66.