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 We cannot use `#not` here. `Order#line_items` does NOT return a `MemoryScope` instance. Its a classic `ActiveRecord::Association` and the whole point here is NOT going to the DB to avoid duplicate loads with lazy heads.