patch 8.1.0397: no event triggered after updating diffs

Problem:    No event triggered after updating diffs.
Solution:   Add the DiffUpdated event.
diff --git a/src/diff.c b/src/diff.c
index 02f1c33..809694a 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -942,6 +942,8 @@
     curwin->w_valid_cursor.lnum = 0;
 
     diff_redraw(TRUE);
+
+    apply_autocmds(EVENT_DIFFUPDATED, NULL, NULL, FALSE, curbuf);
 }
 
 /*