Fix diff after :Gwrite

This works around a bug in Vim that was fixed in patch 8.0.0421.

Closes https://github.com/tpope/vim-fugitive/issues/1102

References https://github.com/tpope/vim-fugitive/pull/859

autoload/fugitive.vim

@@ -4139,6 +4139,7 @@ function! s:WriteCommand(line1, line2, range, count, bang, mods, reg, arg, args)
             silent execute '1,'.last.'delete_'
             silent write!
             silent execute lnum
+            diffupdate
             let did = 1
           finally
             if exists('restorewinnr')