Require merge request to be up-to-date before merging

Description

On GitHub there is the possibility to disable merging of Pull Requests when they are out of date with the base branch.

Proposal

Add a setting to the protected branches which allows to disable merging if the source branch is behind the target branch.

Links / references

https://github.com/blog/2051-protected-branches-and-required-status-checks

/label ~"feature proposal"