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/auevents.lua

@@ -33,6 +33,7 @@ return {
     'CursorHoldI',            -- idem, in Insert mode
     'CursorMoved',            -- cursor was moved
     'CursorMovedI',           -- cursor was moved in Insert mode
+    'DiffUpdated',            -- diffs have been updated
     'DirChanged',             -- directory changed
     'EncodingChanged',        -- after changing the 'encoding' option
     'ExitPre',                -- before exiting

src/nvim/diff.c

@@ -924,6 +924,8 @@ void ex_diffupdate(exarg_T *eap)
   curwin->w_valid_cursor.lnum = 0;
 
   diff_redraw(true);
+
+  apply_autocmds(EVENT_DIFFUPDATED, NULL, NULL, false, curbuf);
 }
 
 ///

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