* lisp/vc/diff-mode.el: Reduce redundancy in diff-syntax-fontify code

* lisp/vc/diff-mode.el: Reduce redundancy in diff-syntax-fontify code

(diff--iterate-hunks): New function extracted from diff--font-lock-refined.
(diff--font-lock-refined, diff--font-lock-syntax): Use it.
(diff--overlay-auto-delete): Rename from diff--font-lock-refine--refresh.
(diff--font-lock-syntax--refresh): Delete.
(diff-syntax-fontify-hunk): Don't completely silence errors.
(diff-syntax-fontify-props): Remove `no-init` arg, testing if `file` is
nil instead.  Adjust all callers.
(diff-syntax-fontify-props): Remove redundant code since we don't
modify the buffer.