fix: refactor and a bit of cleanup

[avidal]

This patch has a bit of refactoring and a single bugfix. Most of the factoring is around making it easy to bring in another command that writes remote commits (see #15 for the prototype which will be adapted to this PR).

The bug that's fixed is around avoiding writing the same commit message trailers in chain situations, where a pipeline copies the previous message and then commit-headless appends an author trailer.

[datadog-datadog-prod-us1]

✅ Code Quality    ✅ Code Vulnerabilities    ✅ Libraries

ℹ️ Info

🎉 All green!

🛠️ No new code quality issues
🛡️ No new code vulnerabilities
📚 No new vulnerable libraries detected

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