atoi() was getting redundant, and unfortunately we can't easily load
helpers.py in some places where we'd want to, because it depends on vars.py.
So move it to its own module.
It used to say:
redo: t/all
redo: hello
and now it says:
redo t/all
redo t/hello
ie. there's no colon, and the path is intact. That means if the build
fails, you can cut-and-paste 'redo t/hello', add a -v, and try to debug
what went wrong.
