unionrepo: get rid of `baserevision` The method is not called anywhere anymore, so we can safely drop it. Some of the comment get moved to `baserevdiff` because we did not got rid of it (yet).

unionrepo: use a lower level overide in unionrepo too The unionrepo class also have a strange `baserevision` hack, let's try to get ride of it too.

bundlerepo: drop the custom `rawdata` implementation We can rely on the main one now.