Fix bug where stable sync is used when sorting is enabled

[arnaudsjs]

No description provided.

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[arnaudsjs]

I also think there is a bug in the _stable_sync() method. When preserve_position==False it can happen that the same type error occurs multiples times in the same module (because the line numbers are set to 0). If that is the case and only once occurrence of the type error is resolved, the added and the removed sets in the _stable_sync() method will be both empty. The result is that no changes will be made to the baseline file. However, when running mypy-baseline filter, it will report a diff. Do you agree this is a bug?

[orsinium]

Thank you!