A VC command that still doesn't allow using diff-mode automatically
thus preventing proper fontification and all other related features
is `vc-root-diff' with a prefix argument HISTORIC.  It has such comment:

      ;; FIXME: this does not work right, `vc-version-diff' ends up
      ;; calling `vc-deduce-fileset' to find the files to diff, and
      ;; that's not what we want here, we want the diff for the VC root dir.

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: