bug#51016: 28.0.50; 'diff-font-lock-prettify' breaks display of outline headers

[Juri Linkov]

>> I updated the patch after a week using it. Outlines headers are now
>> updated for diff comming from Git even when diff-font-lock-prettify is
>> nil.
>
> Thanks for the patch and the thorough case for reproduction.  I've now
> applied it to Emacs 29 (with some trivial changes).

This patch broke vc-diff.  For example, after 'C-x v L' type 'd',
and check that *vc-diff* buffer doesn't have buffer-local variables
diff-vc-backend, diff-vc-revisions anymore, because they were defined
in vc-diff-internal after diff-mode, but now the postponed diff-mode
kills these buffer-local variables:

     (set-buffer buffer)
-    (diff-mode)
     (setq-local diff-vc-backend (car vc-fileset))
     (setq-local diff-vc-revisions (list rev1 rev2))