To create recovery bundle, swh-alter needs to be able to retrieve
full SkippedContent objects from the storage.
The new method skipped_content_find() allows to retrieve all
SkippedContent objects matching a given set of hashes. Usually, this
should only be one, but multiple objects might be returned in case of
hash collisions.
While implementing this, #4693 was identified which prevent testing the implementation with the PostgreSQL storage in the case a SkippedContent references a known origin.
Ref: swh-alter#6 (closed)