loader: Upgrade repository with old formats prior loading
The loader only supports the latest bzr format (2a) so try to upgrade old repository formats to it prior the loading process.
I tested this change in the docker environment and it seems to work, some upgrade can take some times though.
Related to #3945 (closed)
Fixes SWH-LOADER-BZR-3
Migrated from D7202 (view on Phabricator)