Support ido-vertical-mode better

* lisp/ido.el (ido-exhibit):
Prepend a space to INF if it starts with a newline (bug#39379).

lisp/ido.el

@@ -4732,6 +4732,9 @@ For details of keybindings, see `ido-find-file'."
             (delete-overlay ido--overlay))
           (let ((o (make-overlay (point-max) (point-max) nil t t)))
             (when (> (length inf) 0)
+              ;; For hacks that redefine ido-completions function (bug#39379)
+              (when (eq (aref inf 0) ?\n)
+                (setq inf (concat " " inf)))
               (put-text-property 0 1 'cursor t inf))
             (overlay-put o 'after-string inf)
             (setq ido--overlay o)))