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
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate