Implement a merge check to ensure that a pipeline has run against latest version of target

Having merge checks requiring successful builds is great, and the new pull-requests pipeline feature where it does a merge onto the target branch before running the pipeline is also awesome! But there's an enhancement I'd like to see.

Let's say we have two PRs with incompatible changes; both have successful pipelines when run in isolation of each other. One PR is subsequently merged to master. The other PR could also be merged since it has a green build, however it would be awesome to:

1. block merging until a new pipeline is run for the PR, against the new commit on the target branch
2. have a setting to allow automatically triggering this new pipeline