The previous detection mechanism special-cased `canary` which lead to
large diffs when backport branches ran CI.
We stop special casing `canary` reducing complexity and giving us
another flake-detecting attempt when the change is merged.
That way we can safely enable flake detection on backport branches
without having to test large diffs when PRs are merged targetting
backport branches. New backport branches will no longer have flake
detecton and new deploy test runs disabled.
This PR extracts the existing git change detection logic from the `get-changed-tests` script into a separate module. This allows reusing the same logic in the `run-for-change` script, ensuring consistent behavior across the two scripts to determine changed files. Specifically, this ensures that in stacked PRs, the docs-only change detection (and the test-only change detection in the upstack PR) works correctly by diffing against the appropriate base branch.
Sebastian "Sebbie" Silbermann