revlog: return sidedata map from `_revisiondata`
Nothing extra any side data yet. However, it will happens in the future. So we
better prepare the callers of the `_revisiondata` to deal with it.
Differential Revision: https://phab.mercurial-scm.org/D6809
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state