swh-model generates directory entries with bogus permissions

swh.model (and therefore all the loaders that use it) generates directory entry permissions as decimal numbers (eg 100644) instead of the proper value (0o100644 = 33188).

Thankfully, it computes the identifier for the directory with the proper, octal value, so only the database value needs changing - we don't need to recompute identifiers.

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Migrated from T685 (view on Phabricator)