patch 8.2.2870: CmdlineChange event triggered twice for CTRL-R

Problem:    CmdlineChange event triggered twice for CTRL-R.
Solution:   Return CMDLINE_NOT_CHANGED from cmdline_insert_reg().
            (closes #8219)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 5ed283f..bba7e72 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -1810,6 +1810,12 @@
   call feedkeys(":\<F1>\<CR>", 'xt')
   call assert_equal(['l', 'ls'], g:log)
 
+  let @b = 'b'
+  cnoremap <F1> a<C-R>b
+  let g:log = []
+  call feedkeys(":\<F1>\<CR>", 'xt')
+  call assert_equal(['a', 'ab'], g:log)
+
   unlet g:log
   cunmap <F1>
   augroup test