(only if cmd exited successfully) use tmp file as input to sed pipeline.
This works around two issues:
(1) a pathological case where the script would fail in ... interesting
ways if the command being executed closed its stdout. (Certain
commands are used only for their side effects, but not their output,
and doing some testing on my own i got into hot water when one
of my mods caused a command to close its output).
(2) the fact that genassym would succeed even when the command in
fact failed (because the last cmd in the pipeline is the one whose
exit status would be reported).
d45416e707