librarian: `librarian generate` PR includes non-relevant commit messages for unchanged packages

[parthea]

See PR googleapis/google-cloud-python#14671 where the PR body was updated to remove non-relevant commit messages.

Only 8 packages where updated in PR 14671
https://screenshot.googleplex.com/A76Gmo2i7REMxsk

However the PR body contained conventional commit messages for packages where there were no changes. For example, see the following message for google-shopping-merchant-products which was not changed in the PR.

BEGIN_NESTED_COMMIT
fix: [google-shopping-merchant-products] upgrade gRPC service registration func
An update to Go gRPC Protobuf generation will change service registration function signatures to use an interface instead of a concrete type in generated .pb.go service files. This change should affect very few client library users. See release notes advisories in https://github.com/googleapis/google-cloud-go/pull/11025.

PiperOrigin-RevId: 815912123

Source-link: [googleapis/googleapis@6e79e73](https://github.com/googleapis/googleapis/commit/6e79e73)
END_NESTED_COMMIT