Add PR comment when commits are _from the future_

[mspiegel]

The default behavior of checks-out is to only accept approvals that occur at a later time than the HEAD commit on the compare branch. The commits are timestamped by the local clock at the time of commit, not a clock on the GitHub server. If the local clock is significantly into the future then checks-out will ignore all approvals. We can detect this condition and report it as a comment on the pull request.