diff: fix textconv error zombies

diff: fix textconv error zombies

To make the code simpler, run_textconv lumps all of its
error checking into one conditional. However, the
short-circuit means that an error in reading will prevent us
from calling finish_command, leaving a zombie child.
Clean up properly after errors.

Based-on-work-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>