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I would always use "that was," because to my ear, "that's" always implies "that is," and your sentence is in the past tense.
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/15936 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/15936 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/