bug#55800: using rgrep function interactively with fido-mode or fido-vertical-mode causes errors

[Eli Zaretskii]

> From: Yilkal Argaw <yilkalargawworkneh@gmail.com>
> Date: Sun, 5 Jun 2022 10:20:20 +0300
> 
> Start emacs with emacs -q
> M-x fido-vertical-mode                    ;; fido-mode has the same
> effect I tried
> M-x rgrep
> ethiopic                                           ;; the string to search
> .                                                       ;; select
> default=all for file type  by simply clicking just press Enter
> ~/Documents/src_codes/emacs      ;; select the directory to rgrep in
> and press Enter

The problem seems to be in grep-read-files, when fido-mode is used:
the prompt says "default all", but fido understands RET as selecting
the currently-highlighted candidate, which happens to be "./" -- and
that, of course, makes no sense as an argument to "-name".