Feature request: way to search buffer history

- [ 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

Frequently I remove a line of code and need to re-add it. It would be cool to have a way to search vim's undo-tree contents for lines, so that, for example, I could activate a keybinding and type 'import' to see all the imports that have been removed from the current buffer.

In some cases, you would want to search but not immediately add, but rather view - for example when you are searching for a multiline section. In that case, another keybinding might open for example the [MundoDiff](https://github.com/simnalamburt/vim-mundo) of that change to the buffer.

I am not asking you to implement, but would just like to consolidate efforts.