fix(git-node): fix handling of existing trailers

[aduh95]

In nodejs/node#62401, the author has added manually the PR-URL for some reason, which gets filtered out since #1036. However, we had another piece of code that skips adding metadata that was on the original commit message – which was filtered out.

This PR is taking a different approach of removing all the original trailers, adding back the one we won't add ourselves, and finally adding all the "automated ones".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.56%. Comparing base (72e8772) to head (5118f34).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1056   +/-   ##
=======================================
  Coverage   80.56%   80.56%           
=======================================
  Files          39       39           
  Lines        4846     4846           
=======================================
  Hits         3904     3904           
  Misses        942      942           

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