revlog: use `self.sidedata` directly in `revlog.clone`
We want to change `revlog._revisiondata` signature, so we need to migrate users
away.
Differential Revision: https://phab.mercurial-scm.org/D10782
== New Features ==
* `hg config` now has a `--source` option to show where each
configuration value comes from.
== Default Format Change ==
These changes affects newly created repositories (or new clone) done with
Mercurial 5.8.
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==