Add a hook to magit that notifies diff-hl

- as per the diff-hl-mode README
  If you're using a version before 2.4.0, it defines
  magit-revert-buffer-hook (or magit-not-reverted-hook),
  which we use.

  When using Magit 2.4 or newer, add this to your init script:
  (add-hook 'magit-post-refresh-hook 'diff-hl-magit-post-refresh)

core/prelude-editor.el

@@ -392,6 +392,7 @@ indent yanked text (with prefix arg don't indent)."
 ;; diff-hl
 (global-diff-hl-mode +1)
 (add-hook 'dired-mode-hook 'diff-hl-dired-mode)
+(add-hook 'magit-post-refresh-hook 'diff-hl-magit-post-refresh)
 
 ;; easy-kill
 (global-set-key [remap kill-ring-save] 'easy-kill)