Enable Copilot to generate Git commit messages based on staged changes

[khatchad]

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.