svn.loader: Fix latest swh.model.from_disk api change in loader

Prior to this commit, the api change break the current loader implementation. This fixes it by simply adapting the api calls. This will unbreak the current ci build failure [1]

• [1] https://jenkins.softwareheritage.org/job/DLDSVN/job/tests/536/console

Test Plan

tox

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Migrated from D2725 (view on Phabricator)