The previous coding mangled the pathname calculation for
incremental files located in user-defined tablespaces.
Enhance the test cases to cover such cases, as I should have
done originally. Thanks to Andres Freund for alerting me to the
lack of test coverage.
Discussion: http://postgr.es/m/CA+TgmoYdXTjo9iQeoipTccDpWZzvBNS6EndY2uARM+T4yG_yDg@mail.gmail.com