upgrade: add an argument to control manifest upgrade The argument can be used to only "clone" manifest revlog or clone all of them but this one. The selection will make more sense once we have a `--changelog` flag in the next changesets.

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

unionrepo: drop `baserevdiff` It has no caller anymore.

unionrepo: use normal inheritance scheme to call revdiff

unionrepo: fix `revdiff` implementation to use `rawdata` The parent code is using rawdata so we should use it here. Before this change, union repo was probably broken with some flag processors.

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).