vim-patch:8.1.0397: no event triggered after updating diffs

Problem:    No event triggered after updating diffs.
Solution:   Add the DiffUpdated event.

e8fa05b5bc

src/nvim/testdir/test_diffmode.vim

@@ -2,6 +2,9 @@
 
 func Test_diff_fold_sync()
   enew!
+  let g:update_count = 0
+  au DiffUpdated * let g:update_count += 1
+
   let l = range(50)
   call setline(1, l)
   diffthis
@@ -27,6 +30,9 @@ func Test_diff_fold_sync()
   call win_gotoid(winone)
   call assert_equal(23, getcurpos()[1])
 
+  call assert_equal(1, g:update_count)
+  au! DiffUpdated
+
   windo diffoff
   close!
   set nomodified