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

[Juri Linkov]

>> I suggest to restore the original call to `diff-mode` (before the
>> asynchronous operation completes) and keep the initialization of
>> `diff-buffer-type` by searching but make it available to
>> `vc-diff-finish' which is called when the asynchronous operation
>> completes.
>>
>> See the attached patch for details.
>
> Thanks; this fixes the problem reported by Juri (as far as I can see
> after doing some light testing), so I've now pushed the patch to Emacs
> 29.

I confirm it's fixed (Matthias placed two counterpart functions together:
diff-setup-whitespace and diff-setup-buffer-type in the patch,
so I restored the same in master for readability).