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test(pr,stack): drive the lease, and ask the URL the push uses master

A `pushInsteadOf` rewrite makes the fetch URL a different machine, so the head a push is about to overwrite has to be read from `remote get-url --push`; the null OID is no expectation at all and now yields none. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Change-Id: Iefdbb6fa0da9d0af25bdb11bf658255c55f361be


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7 changed files