Fix flaky "t5516.81 deny fetch unreachable SHA1, allowtipsha1inwant=false" #240
Description
This test seems to fail infrequently in our Azure Pipeline, apparently most often in the msvc job. See https://dev.azure.com/Git-for-Windows/git/_build/results?buildId=36820&view=ms.vss-test-web.build-test-results-tab for an example.
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