vim: co<space> to toggle 'iwhite' in diff-mode

- dO: already provided by unimpaired.vim
 -give up on 'noshelltemp'
  https://github.com/tpope/vim-sensible/issues/73
  https://github.com/chrisbra/Recover.vim/issues/30
- c_CTRL-W c_CTRL-U