Also only use unique bug numbers in generated commit message.

NOTRY=true

Bug: v8:6482
Change-Id: Ie0d14640053bacb907d18e6ccb5b4d0b6ecbf661
Reviewed-on: https://chromium-review.googlesource.com/532914Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45899}