Provide :DiffOrig

This seems like a weirdly specific thing to provide in a set of
defaults, but since defaults.vim provides it I guess we can too.

References: https://github.com/tpope/vim-sensible/issues/86

plugin/sensible.vim

@@ -103,6 +103,12 @@ if empty(mapcheck('<C-W>', 'i'))
   inoremap <C-W> <C-G>u<C-W>
 endif
 
+" From `:help :DiffOrig`.
+if exists(":DiffOrig") != 2
+  command DiffOrig vert new | set bt=nofile | r ++edit # | 0d_
+        \ | diffthis | wincmd p | diffthis
+endif
+
 " Load matchit.vim, but only if the user hasn't installed a newer version.
 if !exists('g:loaded_matchit') && findfile('plugin/matchit.vim', &rtp) ==# ''
   runtime! macros/matchit.vim