Add support for including fixed issues from the tag commit

We do not create specific tag commits. Instead, we add a tag to the commit we want to create the release from. When doing so, and creating a changelog from gren, we've noticed that the fixed issue from the tag commit isn't included in the changelog.

Would it be possible to add a way to include the fixed issues in the tag commit as well?

Thanks.

No files or tests are named. Start by tracing how gren creates a changelog from a tag and identifies fixed issues, then reproduce the case where the tag is applied directly to an existing commit; done means the fixed issue from that tag commit appears in the changelog.

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