Add a confirmation dialog that pops up when a user trying to merge a PR that is out of date

[a-asharapova]

As a user, I want to see a confirmation dialog when I press the Merge button on an out-of-date PR.

[emmerich]

That's a cool idea. We're planning on building more controls around change requests (e.g. being able to say "every CR requires one approval"). Would having a "require all change requests to be up-to-date" be a good solution to this? That way it fits well in our approach for CR checks.

[a-asharapova]

@emmerich It would, but ideally admins should be able to disable and enable this check.