patch 8.2.2643: various code not covered by tests

Problem:    Various code not covered by tests.
Solution:   Add a few more test. (Yegappan Lakshmanan, closes #7995)
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index f12e152..2621412 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -747,4 +747,13 @@
   bwipe!
 endfunc
 
+" Test for redo in insert mode using CTRL-O with multibyte characters
+func Test_redo_multibyte_in_insert_mode()
+  new
+  call feedkeys("a\<C-K>ft", 'xt')
+  call feedkeys("uiHe\<C-O>.llo", 'xt')
+  call assert_equal("He\ufb05llo", getline(1))
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab