fix: do not add trailing commit the last positional parameter already…

[a-wallen]

… has one

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[copybara-service]

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[a-wallen]

https://critique.corp.google.com/cl/695463353

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/394660 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/394660 has been updated with the latest commits from this pull request.