bug#74208: 31.0.50; minibuffer read-file-name-default mutates global value of default-directory incorrectly

[Madhu]

minibuffer.el: (read-file-name-default) has the following code:
```
                     (minibuffer-with-setup-hook
                         (lambda ()
                           (setq default-directory dir)
                   ...
                          (set-syntax-table minibuffer-local-filename-syntax))
```

This mutates the global binding of default-directory which is incorrect.

To demonstrate the problem, in emacs -Q

(insert "http://example.com";)
(setq enable-recursive-minibuffers t)

position point at in the middle of the string  http://example.com
M-: (ffap)

;; while waiting for input
M-: default-directory
;; => default-directory is bound to "http://example.com";
M-: (shell-command "echo foo")
; ;=>
;; Debugger entered--Lisp error: (file-missing "Setting current directory" "No 
such file or directory" "http://example.com/";)

I think this can be addressed by binding default-directory before
modifying it.

```
diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -3671,6 +3671,7 @@ read-file-name-default
                               (expand-file-name dir))))
                     (minibuffer-with-setup-hook
                         (lambda ()
+                         (let ((default-directory default-directory))
                           (setq default-directory dir)
                           ;; When the first default in `minibuffer-default'
                           ;; duplicates initial input `insdef',
@@ -3689,7 +3690,7 @@ read-file-name-default
                                  (with-current-buffer
                                      (window-buffer 
(minibuffer-selected-window))
                                   (read-file-name--defaults dir initial))))
-                         (set-syntax-table minibuffer-local-filename-syntax))
+                         (set-syntax-table minibuffer-local-filename-syntax)))
                       (completing-read prompt 'read-file-name-internal
                                        pred require-match insdef
                                        'file-name-history default-filename)))
```

[I'm sending this before I forget so it can be reviewed, but I can
send this patch as as attachment if needed, and it'll take some more
time until I figure out how to write a test for it and test it.]