* (bug 7872) Deleted revisions can now be viewed as diffs showing changes

* (bug 7872) Deleted revisions can now be viewed as diffs showing changes
  against the previous revision, whether currently deleted or live.
In cases of mixed or merged histories this may not produce ideal results.
Currently only auto-selected previous revision is available to diff against; in future could potentially allow diffing against any other revision.
'Show changes' button may currently display on the individual revision view form even if there's no prior revision available. Should be correct in the list view though.