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 T3945
Fixes [SWH-LOADER-BZR-3](https://sentry.softwareheritage.org/share/issue/0d40e46939854141afdfd1c04a742915/)