bug#33650: 27.0.50; Root diffs between revisions

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Thu, 06 Dec 2018 23:54:16 +0200
> 
> This patch fixes this omission by implementing `vc-root-version-diff'
> that is able to compare two revisions even on separate branches from
> their roots:

Thanks.  A few comments:

> +(defun vc-root-version-diff (_files rev1 rev2)
> +  "Report diffs between revisions of the whole tree in the repository 
> history."

This doc string doesn't describe the arguments, and doesn't tell how
those arguments are populated in interactive invocations.

I also wonder what would this mean for a VCS such as CVS or RCS, where
there's no meaning of a "tree revision".  I'm guessing this is why the
old code deduced files to diff.  See also the description of vc-diff
in the Emacs manual.

Finally, we need documentation changes to go with this change of
user-visible behavior.