Current-bench gives up testing commits on the master branch if new commits have been pushed in the meantime

[kit-ty-kate]

To reproduce:

• Merge a PR (this triggers current-bench on the merge commit)
• While current-bench is still running, merge another PR

This will cancel the bench on the previous merge commit an leave the github status for said merge commit as "pending" for all eternity.

[punchagan]

On PRs, cancelling the previous run may still be a good idea, but on master we still want to benchmark for those commits, I think. Leaving the status as pending is a bug, in any case.