patch 8.2.5166: test for DiffUpdated fails

Problem:    Test for DiffUpdated fails.
Solution:   Also accept a count of two.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 4c7aff5..dcacd55 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ