[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
Behaviour of the build should change according to causes (e.g. a build triggered by a git push may perform different initial operations from the same job triggered by a manual execution [not a CVS example, I know]). However this code here does feel wrong in how it's trying to achieve the end result, but I've not really looked at an alternative.