Don't re-run CI in the merge queue if branch is already up to date

[ejfine]

Is there an existing issue for this?

• I have searched the existing issues

Feature description

A way to check the git sha and determine if the code after the merge will be the same as the tip of the branch that CI already ran on as part of the PR process. If so, then immediately exit the CI pipeline (that was triggered by the merge_group event). Thereby avoiding an unnecessary run of the full CI pipeline in the merge queue.

I recall in the past @zendern was able to figure this out, so it's definitely possible 🤞