When auto-choosing a .redo dir, prefer ones uphill that already exist.

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.

clean.do

@@ -1,2 +1,3 @@
 rm -f t/hello t/[by]ellow t/*.o *~ .*~ t/*~ t/.*~ *.pyc t/CC t/LD
-rm -rf .redo
+rm -rf .redo t/.redo
+