Intermittent fatal error when multithreaded

When attempting to convert cvs-fast-export's own tests/t9601.testrepo,

find tests/t9601.testrepo -name '*,v' | sort | cvs-fast-export >/dev/null

intermittently fails with:

cvs-fast-export fatal: child commit emitted before parent exists

Other times the same command works as expected.

Passing --threads 1, -F, or both makes it succeed every time.

Of the CVS repositories in cvs-fast-exports's tests subdirectory, t9601.testrepo is the only one that shows this behaviour. The rest seem to be deterministic (at least as far as stderr messages are concerned; I haven't attempted to test whether the resulting fast-import streams are always correct).

Tested against both 1.43 and master (065fc0aa).