GitBranchFiles ?

- [x] I have fzf 0.30.0 or above
- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [x] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [x] I have read through the manual page of fzf (`man fzf`)
- [x] I have searched through the existing issues

This is pretty New and different from GitFiles, let say i want to get changes made on all the files for the current branch am on.
Not talking about GitFiles[?]
But for actually all the files on the current branch that were changed from the base branch.

You can obtain this by using :
- On Files changed that differs from the base branch : `git diff --stat $(git merge-base HEAD origin) --name-only`
- And to get changed per files : ` --patch -- this_file.py`

I have started a [draft POC](https://github.com/junegunn/fzf.vim/pull/1539) and i was wondering if i can propose a well clean PR for that ?