Add other filetypes to `set bufhidden=delete` autocmd

[cstyles]

Running git config --edit or git rebase --interactive will open a buffer in neovim with a filetype of gitconfig or gitrebase respectively. We want these buffers to be deleted automatically when they're hidden just like buffers with filetype gitcommit so let's add them to the relevant note in the README.

[mhinz]

Thanks! ✨