What: delete update_custody.spawn_kwargs (no production caller since #132927 removed the gc fold) and
its helper-only test; a parametrized test drives run_git itself (gc holds the lock fd, a reader not).
Why: the old test exercised only the dead helper and returned early on win32, a false green.
Red/green: mutant 'run_git raises' -> new test 2 failed (old test passed under it); real code -> 8 passed.
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