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ARROW-8872: [CI] Restore ci/detect-changes.py

This is needed for Travis-CI builds to function properly.
The file was mistakingly removed in db1c49a77f78152d442299082f6a663e59f91174, part of PR #7080 (fixing ARROW-8662).

Closes #7234 from pitrou/ARROW-8872-ci-detect-changes

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>