Investigate and fix snapshots with broken links

We have a quantity of snapshots with broken links of some sorts:

• branches with target_type = 'revision' but actually pointing at releases, directories or contents
• branches with target_type = 'revision', with the revision that is missing

The first series is due to the original git loader not knowing how to distinguish branches pointing to different kinds of objects. The second series is due to other bugs, where snapshots have been loaded with the underlying data not being loaded yet.

This task will track how these snapshots are getting fixed.

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Migrated from T995 (view on Phabricator)