storage.in_memory: Fix origin_list implementation
This kept on creating unrelated errors on other modules (see swh-scheduler!317 (closed) for the most recent but not only)
Related to T645
Test Plan
tox
Migrated from D3683 (view on Phabricator)
This kept on creating unrelated errors on other modules (see swh-scheduler!317 (closed) for the most recent but not only)
Related to T645
tox
Migrated from D3683 (view on Phabricator)