It turns out my patch doesn't fully fix the issue, when doing completion
at the end of a file path.  For example:

1. C-x C-f ~/src/emacs/emacs-29/lisp/.el
2. TAB to trigger completion, moving point to before .el
3. M-<down>
4. The filenames are inserted before the .el, so one gets for example
   ~/src/emacs/emacs-29/lisp/abbrev.el.el

The attached patch for the Emacs 29 branch fixes this remaining case.