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

...haviour 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
@mc1arke what do you think of this? It feels wrong to me; the behavior of a build should not vary according to its `Cause`s, which are normally considered to be informational only.