Workaround / in tree entry names

As far as I know, these only happens in https://github.com/jkingok/sl4a-scripts/

However, we ingested that repository before we started rejecting / from Directory entry names (swh-model!204 (closed)); so we had to retroactively "fix" the three "bad" directories that came from this repo in the DB and journal (by replacing / with _ and adding a raw_manifest), because it crashes any script that tries to read them: T4644

For the sake of repeatability/consistency, we should update the git loader to do the same transformation.

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Migrated from T4660 (view on Phabricator)