Some objects from the original GitHub import have never actually been imported.

For around 20.000 origins that have been imported in the original github run, we failed to actually import the data, but we still created occurrences referencing commits (that are now dangling).

When doing incremental updates over these repositories, the git loader assumed that the revisions the occurrences pointed to were indeed imported, and therefore we have never filled the gaps by reimporting the data.

We should reimport it from the original git clones.

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Migrated from T846 (view on Phabricator)