Fix diff sometimes opening in preview window

If the previous window no longer exists when Gedit is called, the
attempt to change windows with 'wincmd p' fails and 'wincmd w' should be
used instead.

plugin/fugitive.vim

@@ -1336,8 +1336,10 @@ function! s:Edit(cmd,bang,...) abort
       if winnr('$') == 1
         let tabs = (&go =~# 'e' || !has('gui_running')) && &stal && (tabpagenr('$') >= &stal)
         execute 'rightbelow' (&lines - &previewheight - &cmdheight - tabs - 1 - !!&laststatus).'new'
-      else
+      elseif winnr('#')
         wincmd p
+      else
+        wincmd w
       endif
       if &diff
         let mywinnr = winnr()