minor, support custom diff(1)'s through --diff switch

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, support diff(1) tool specifying through DIFF env var too

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'feat/diff'

* feat/diff:
  minor, support diff(1) tool specifying through DIFF env var too
  minor, support custom diff(1)'s through --diff switch