Currently GitLab backup restoration works on exactly the same GitLab
version, as the one with which the backup was made:

    https://gitlab.com/gitlab-org/gitlab-ce/blob/7383453b/lib/backup/manager.rb#L132

However in many cases restoring backup on a newer GitLab version is
desirable - e.g. when moving GitLab instance to upgraded software.
GitLab answer - that we should first prepare exactly the same GitLab
version on moved instance, restore backup, then upgrade GitLab itself
_inplace_, is not satisfactory in e.g. slapos case - as upgrading can
take a long time, and in-place software changes can render GitLab
instance non-working.

What we better prefer to do is to fully prepare new GitLab software
version, and then knowing software is ready, restore backup in a quick
manner.

The following analysis says we should be 99% ok to do so:

1. git-backup cares backward compatibility for format of repositories backup.
2. db dump is backward compatible, because Rails, when seeing old db
   schema, will run migrations.
3. the rest is relatively minor - e.g. uploads, which is just files in
   tar, and format for such things changes seldomly.

because of 3, strictly speaking, it is not 100% correct to restore
backup from older gitlab version to newer one (since gitlab does not
provide a promise of backward compatibility on e.g. uploads/ backup
format) , but in practice it is 99% correct and is usually handy.

/cc @kazuhiko