chore, allow stderr output from diff subprocess

Signed-off-by: Sebastian Freundt <hroptatyr@fresse.org>
fix, unset feeder pid in differ() when feeder isn't used

Signed-off-by: Sebastian Freundt <hroptatyr@fresse.org>
Merge branch 'code/hygiene'

* code/hygiene:
  fix, unset feeder pid in differ() when feeder isn't used
  chore, allow stderr output from diff subprocess