Since there are no strict rules about redo behaviour, here are some
remarks about goredo's one:

* "all" target is default
* stdout is always captured, but no target is created if it was empty
* non-existent targets are considered always out of date
* .do's $3 is relative path to the file in target directory
* .do search goes up to / by default, but can be limited with either
  $REDO_TOP_DIR environment variable, or by having .redo/top file in it
* target's completion messages are written after they finish
* executable .do is run as is, non-executable is run with: /bin/sh -e
* tracing (-x) can be obviously done only for non-executable .do
* Parallelizable build is done only during redo-ifchange for human
  convenience: you can globally enable $REDO_JOBS, but still do for
  example: redo htmls infos index upload