[ci] Improve change detection logic in run-for-change script
#85619
+104
−108
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This PR extracts the existing git change detection logic from the
get-changed-testsscript into a separate module. This allows reusing the same logic in therun-for-changescript, 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.