[ZD-19670] CloudBees customer reported that the quite period checkout behaviour has anomolous logic that can result in failed builds

- This is likely where there is a delay between the build leaving the quiet period and the build starting
- If there is a commit during that period of time then the build may checkout an inconsistent state
- The solution is to use the time when the quiet period is completed as the timestamp for the checkout
- Obviously people may prefer the original behaviour in some cases
Merge pull request #32 from jenkinsci/quiet-period-exit-aware-checkout

[ZD-19670] CloudBees customer reported that the quite period checkout be...