redo-ifchange already does this, and then calls redo.py, so it's redundant.
The only weird case is if a .do file calls 'redo' instead of
'redo-ifchange'. I'm not sure what exactly is supposed to happen here.
test.do does this, for example, and it works, but only because we always do
'redo test' and nobody depends on 'redo-ifchange test'; if they did, the
dependencies would be off, I guess. But maybe it's meaningless to do that
anyway.
So if we're in redo/t/ and running 'redo hello', we'll detect redo/.redo and
use that one rather than creating a new redo/t/.redo directory.
The downside of this is we get slightly different behaviour if the *first*
thing you build isn't from the root. Probably that's bad, but it should
hopefully be rare.
And move test scripts into t/ subdir to keep things clean.
As part of that, make sure redo adds itself to the PATH on startup so that
subscripts can find it.
