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
diff --git a/src/version.c b/src/version.c
index 8e396cd..3422e7a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -736,6 +736,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    5166,
+/**/
     5165,
 /**/
     5164,