minor, fix, PEBKAC, fan out to hxdiff(1) whatever is readable or writable

Before we just went round-robin, assuming diff(1) would consume data from
both input descriptors in a round-robin fashion too.  Boy were we wrong.
test, provide regressions for stalling diff bug in hxdiff(1)

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'bug/stalling-pipes'

* bug/stalling-pipes:
  test, provide regressions for stalling diff bug in hxdiff(1)
  minor, fix, PEBKAC, fan out to hxdiff(1) whatever is readable or writable