upgrade: allow upgrade to repository using sidedata
Repository can now be migrated to support sidedata. More requirements and
migration will be needed to actual side-data usage. This is a step in that
direction.
To test the feature, we leverage the test extension. It make sure the `update`
part of the side-data migration actually works.
Differential Revision: https://phab.mercurial-scm.org/D6942
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors