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 https://github.com/unjs/changelogen/issues/62

Validations

• [X] Follow our Code of Conduct
• [X] Read the Contributing Guide.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.