Better support for nil enable-local-variables in vc-find-revision-no-save

Better support for nil enable-local-variables in vc-find-revision-no-save

* lisp/vc/diff-mode.el (diff-syntax-fontify-props):
* lisp/vc/vc.el (vc-find-revision-no-save):
Use enable-local-variables as is only when it's one of these values:
:safe, :all, or nil.  Otherwise, for all remaining values that query,
use :safe.  Also use non-nil 'find-file' arg of 'normal-mode'.
https://lists.gnu.org/archive/html/emacs-devel/2025-02/msg00897.html

(cherry picked from commit 24e8477aa2f5ef228107707078735ade31e5d1c3)