vim-patch:8.2.4899: with latin1 encoding CTRL-W might go before the cmdline

Problem:    With latin1 encoding CTRL-W might go before the start of the
            command line.
Solution:   Check already being at the start of the command line.
https://github.com/vim/vim/commit/ef02f16609ff0a26ffc6e20263523424980898fe

src/nvim/testdir/test_cmdline.vim

@@ -642,6 +642,9 @@ func Test_cmdline_remove_char()
 
     call feedkeys(":abc def\<S-Left>\<C-U>\<C-B>\"\<CR>", 'tx')
     call assert_equal('"def', @:, e)
+
+    " This was going before the start in latin1.
+    call feedkeys(": \<C-W>\<CR>", 'tx')
   endfor
 
   let &encoding = encoding_save