Buildbot no longer supports Python 2.7 on the Buildbot master. GitHub

class buildbot.steps.source.github.GitHub

GitHub step is exactly like the Git step, except that it will ignore the revision sent by GitHub change hook, and rather take the branch if the branch ends with /merge.

This allows to test github pull requests merged directly into the mainline.

GitHub indeed provides refs/origin/pull/NNN/merge on top of refs/origin/pull/NNN/head which is a magic ref that always create a merge commit to the latest version of the mainline (i.e. the target branch for the pull request).

The revision in the GitHub event points to /head is important for the GitHub reporter as this is the revision that will be tagged with a CI status when the build is finished.

If you want to use Trigger to create sub tests and want to have the GitHub reporter still update the original revision, make sure you set updateSourceStamp=False in the Trigger configuration.