Avoid duplicate loads on creating item adjustments

This reduces a source of duplicate records loads that can lead do deadlocks and identity problems.

We oversaw this one on the deduploads optimization branch.

After merging I'm gonna squash this commit into the deduploads one on the next rebase.
@dkubb I think this requires a query builder that is more complex than I want to introduce to `MemoryScope`. I'm happy to revisit this in case we find a 2nd call side it would simplify. But only for one call side I'm fine with `Enumerable#reject`.