svn.loader: Fix uneventful visit case
It was unexpectedly "eventftul" so far [1]. Most probably since [2] [3] or something near.
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[2] https://forge.softwareheritage.org/swh/devel/swh-loader-core!356
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[3] https://forge.softwareheritage.org/swh/devel/swh-loader-core!357
Test Plan
tox
Migrated from D3413 (view on Phabricator)