Migrate undo-tree to use-package

First use of :diminish (and diminish.el) and conditional loading via :if

init.el

@@ -328,9 +328,10 @@ and eval it."
 ;; (ispell-change-dictionary "en")
 (setq ispell-dictionary "en")
 
-;; Comment out if using Evil, uncomment if not
-(require 'undo-tree)
-(global-undo-tree-mode)
+(use-package undo-tree
+  :if (not (featurep 'evil))
+  :diminish undo-tree-mode
+  :init (global-undo-tree-mode))
 
 (with-eval-after-load 'text-mode
   (define-key text-mode-map (kbd "M-p") 'org-drag-line-backward)