The F1 fix runs every updater git mutation with core.hooksPath=<null device>, so the
pre-commit hook this test used to stall `_git_run`'s git never ran and the owner exited
before it could be killed mid-write. R2's invariant is about git and whatever it runs, not
hooks: a clean filter on `git add` (a local mutator) stalls git the same way and inherits the
same lock fd. Still red when `add` does not receive the fd ("a contender owns the checkout
while git still writes").
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