Stacklevel too deep during deepMerge (when calling changeset.unexecute())

## Version

ember-changeset@4.1.0

## Test Case

I've created a test here: #653

## Steps to reproduce

- Create a changeset from an ember-data model with sync belongsTo
- Change this belongsTo
- call changeset.execute()
- call changeset.unexecute() => exception

## Expected Behavior

I was expecting the changeset to have the previous belongsTo model associated.

## Actual Behavior

It seems like ember-changeset should work well with Ember-Data, but I think I do somehting wrong, because it crashes during `unexecute`, when it encounters a belongsTo relation.

```
Error: Unable to 'mergeDeep' with your data. Are you trying to merge two ember-data objects? Please file an issue with ember-changeset.
at Proxy.mergeDeep (merge-deep.js:161:1)
at Proxy.unexecute (validated-changeset.es5.js:409:155)
```
but I don't do any merge by myself

Start with the test case referenced in issue #653, then inspect merge-deep.js at the failing mergeDeep call and validated-changeset.es5.js around unexecute. Reproduce the belongsTo scenario and verify that unexecute restores the previous associated model without the stack overflow or mergeDeep error.

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