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The bots will automatically detect if the commit in a backport pull request is a clean backport. If it is then bots will add the label "clean" to the pull requests. There is also a user command /clean, which can be used to manually label a backport as clean. Backport pull requests which commit is classified as a clean backport can be integrated without reviewers (configurable per repository). This means that not all commit messages for backport commits will have a "Reviewed-by" line.
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Overview
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