Handle reformatting changlogs without sub-headings (changed, fixed etc.)

[mattwynne]

👓 What did you see?

e.g. https://github.com/cucumber/multi_test/commit/a5604e281e217475c5606f26f01dae1ae929d326

✅ What did you expect to see?

Either:

• An error saying to fix your changelog
• Automatically add those entries to a "Changed" subheading

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