fix collection notifications reporting modifications when sort/distinct has been applied

[ironage]

When checking if an item has been modified in a collection, we should check the old indices reported by modifications instead of modifications_new. They will normally be the same, except on sorted collections, when the modification results in a change of the ordering.

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)

[tgoyne]

Using modifications_new rather than modifications should also produce incorrect results on unsorted collections when rows are deleted before the modified rows.

[DominicFrei]

Would it make sense to also check modifications_new whenever modifications gets checked? Just to make sure all four categories do look exactly as expected.

[ironage]

Updated!

[ironage]

Using modifications_new rather than modifications should also produce incorrect results on unsorted collections when rows are deleted before the modified rows.

I added a test for this and unsorted collections appear to be ok. Turns out it is only collections with sort/distinct applied because otherwise a different code path is used: https://github.com/realm/realm-core/blob/master/src/realm/object-store/impl/results_notifier.cpp#L298-L300