>> > Finally, we need documentation changes to go with this change of
>> > user-visible behavior.
>>
>> This feature is already documented in (info "(emacs) Old Revisions")
>>
>>   ‘C-x v D’
>>        Compare the entire working tree to the revision you started from
>>        (‘vc-root-diff’).  With a prefix argument, prompt for two revisions
>>        and compare their trees.
>>
>> It didn't work as documented before properly implemented by the patch.
>
> Isn't the result user-visible change in behavior?

Yes, this fix changes user-visible behavior, so perhaps it should be
mentioned in NEWS.

>>  (defun vc-version-diff (_files rev1 rev2)
>> -  "Report diffs between revisions of the fileset in the repository history."
>> +  "Report diffs between revisions of the fileset in the repository history.
>> +REV1 is an older revision, REV2 is a newer revision."
>
> We usually mention the arguments in the first line of the doc string.
> Something like this:
>
>   Report diffs between revisions REV1 and REV2 in the repository history.
>
> I wonder whether "fileset" needs to be mentioned at all, given your
> changes.  When does any "fileset" come into play here?

"fileset" is essential to be mentioned in `vc-version-diff'
whereas the text "in the repository history" is unnecessary.

>> +(defun vc-root-version-diff (_files rev1 rev2)
>> +  "Report diffs between revisions of the whole tree in the repository history.
>> +REV1 is an older revision, REV2 is a newer revision."
>
> Same here: please mention the arguments in the first sentence.

As "fileset" is essential for `vc-version-diff',
"whole tree" is essential for `vc-root-version-diff':