minibuffer-force-complete-and-exit: Allow input with no matches

* lisp/minibuffer.el (minibuffer--require-match): New variable.
(completing-read-default): Bind it to the REQUIRE-MATCH value.
(minibuffer-force-complete-and-exit): Consult it to allow input
with no matches when a match is not required (bug#38992).

* lisp/icomplete.el (icomplete-exhibit): Use it to render the
correct parens around matches.

lisp/icomplete.el

@@ -541,7 +541,7 @@ See `icomplete-mode' and `minibuffer-setup-hook'."
                           (icomplete--completion-table)
                           (icomplete--completion-predicate)
                           (if (window-minibuffer-p)
-                              (not minibuffer-completion-confirm)))))
+                              (eq minibuffer--require-match t)))))
                  (buffer-undo-list t)
                  deactivate-mark)
             ;; Do nothing if while-no-input was aborted.

lisp/minibuffer.el

@@ -1400,7 +1400,11 @@ scroll the window of possible completions."
    (minibuffer-prompt-end) (point-max) #'exit-minibuffer
    ;; If the previous completion completed to an element which fails
    ;; test-completion, then we shouldn't exit, but that should be rare.
-   (lambda () (minibuffer-message "Incomplete"))))
+   (lambda ()
+     (if minibuffer--require-match
+         (minibuffer-message "Incomplete")
+       ;; If a match is not required, exit after all.
+       (exit-minibuffer)))))
 
 (defun minibuffer-force-complete (&optional start end dont-cycle)
   "Complete the minibuffer to an exact match.
@@ -1464,6 +1468,9 @@ DONT-CYCLE tells the function not to setup cycling."
   "List of commands which cause an immediately following
 `minibuffer-complete-and-exit' to ask for extra confirmation.")
 
+(defvar minibuffer--require-match nil
+  "Value of REQUIRE-MATCH passed to `completing-read'.")
+
 (defun minibuffer-complete-and-exit ()
   "Exit if the minibuffer contains a valid completion.
 Otherwise, try to complete the minibuffer contents.  If
@@ -3748,8 +3755,10 @@ See `completing-read' for the meaning of the arguments."
 
   (let* ((minibuffer-completion-table collection)
          (minibuffer-completion-predicate predicate)
+         ;; FIXME: Remove/rename this var, see the next one.
          (minibuffer-completion-confirm (unless (eq require-match t)
                                           require-match))
+         (minibuffer--require-match require-match)
          (base-keymap (if require-match
                          minibuffer-local-must-match-map
                         minibuffer-local-completion-map))