When diff-refine overlays take precedence over hi-lock text properties,
then hi-lock can't be used to review diffs.  Hi-lock provides two modes:
font-lock highlighting and highlighting with overlays.  Using overlays
solves the problem.  But how to decide when overlays should be used?
Detecting whether the buffer is read-only, so no more text is expected
to be added to the buffer?  Then add overlays instead of using font-lock?
But this heuristics doesn't work in read-only *vc-change-log* buffers
where expanding commits inserts new text that should be highlighted
with font-lock.

Thus the following patch adds a new variable hi-lock-use-overlays
that can be used like:

#+begin_src emacs-lisp
(add-hook 'diff-mode-hook
            (lambda ()
              (setq-local hi-lock-use-overlays t)))
#+end_src