Add an option to use only merge commits

[Barbapapazes]

Clear and concise description of the problem

I wish the changelog could be generated using only merge commit (git log --merges) to have a tool more inclusive for those want do not want use squash merges.

Suggested solution

Add an option which add the --merges to git commands.

Alternative

No response

Additional context

Not sure if this issue must be here or at unjs/changelogen#62

Validations

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• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.