Allow global option for failing if branch isn't up to date with the source branch

[jlaustill]

Is your feature request related to a problem? Please describe.
Yes, when we rolled gitstream out our devs were creating PR's against stale source branches that didn't have the gitstream file. This will be a recurring issues as we roll out new versions of the gitstream file as well. We would like to start depending on gitstream to enforce certain behaviors, but we need to be able to depend on it. This would be a healthy behavior to start with I think.

Describe the solution you'd like
At the time a dev pushes and update to or creates a PR, if their branch isn't up to date with the source branch, simply fail and don't even look for the gitstream file with an appropriate message. Thus ensuring the most update to date gitstream file is always used.

Describe alternatives you've considered
The only other alternative I found was requiring fast forward merging in github, but this has the downside that devs have to constantly merge the last from the source branch to be able to merge.

Additional context
I feel like this feature would be a require fast forward merging "lite" where it only enforces it at the time of the push, but anything merged after that point to the source branch wouldn't block from merging unless there were conflicts.