* lisp/files.el (get-free-disk-space): Use file-size-human-readable

Since there is no longer an option to do:

    (setq directory-free-space-args "-Pmh")

lisp/files.el

@@ -6705,7 +6705,7 @@ If DIR's free space cannot be obtained, this function returns nil."
   (save-match-data
     (let ((avail (nth 2 (file-system-info dir))))
       (if avail
-	  (format "%.0f" (/ avail 1024))))))
+          (file-size-human-readable avail)))))
 
 ;; The following expression replaces `dired-move-to-filename-regexp'.
 (defvar directory-listing-before-filename-regexp