patch 8.2.0670: cannot change window when evaluating 'completefunc'

Problem:    Cannot change window when evaluating 'completefunc'.
Solution:   Make a difference between not changing text or buffers and also
            not changing window.
diff --git a/src/testdir/test_edit.vim b/src/testdir/test_edit.vim
index a9ba31d..f0bf5cc 100644
--- a/src/testdir/test_edit.vim
+++ b/src/testdir/test_edit.vim
@@ -927,7 +927,7 @@
   set completefunc=CompleteFunc
   call setline(1, ['', 'abcd', ''])
   2d
-  call assert_fails("normal 2G$a\<C-X>\<C-U>", 'E565:')
+  call assert_fails("normal 2G$a\<C-X>\<C-U>", 'E578:')
   bwipe!
 endfunc