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