feat(git-node): refactor metadata generation

[aduh95]

Sometimes Node.js contributors add a PR-URL to their commit message (e.g. because the commit is cherry-picked from a different PR), and NCU may end up generating a commit referencing multiple PR-URLs. It's not ideal, in nodejs/core-validate-commit#125 I've explored the idea of making the CI fail on those, but it seems easier to have NCU ignore the user provided trailers when it's going to add the correct ones anyway.

A consequence of that is that commits backported with a manual PR would now list the reviewers of the backport PR rather than the original PR, which seems more logical to me anyway.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.62%. Comparing base (2ea7b22) to head (ea8e744).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1036      +/-   ##
==========================================
- Coverage   80.94%   80.62%   -0.33%     
==========================================
  Files          39       39              
  Lines        4829     4821       -8     
==========================================
- Hits         3909     3887      -22     
- Misses        920      934      +14     

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