Some directories can not be browsed in a release context

When a snapshot contains more than 1000 branches and one tries to browse a directory in a release context, a 404 error is currently raised, see https://archive.softwareheritage.org/browse/origin/directory/?origin_url=https://github.com/python/cpython&release=v3.9.0a4 for instance.

This bug is related to the retrieval of release branch when the snapshot content is large and must be fixed.

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Migrated from T2437 (view on Phabricator)