minimal/do: didn't work correctly with multi-level auto dir creation.

If we tried to build target a/b/c/d and a/b/c didn't exist yet, we
would correctly name the temp file something like a__b/c__d.tmp.  But
if a/b didn't exist yet, we named the temp file a/b__c/d.tmp, which
didn't work.  Instead, name it a/b__c__d.tmp as expected.

clean.do

@@ -1,3 +1,4 @@
+exec >&2
 rm -rf t/.redo redo-sh
 if [ -e .do_built ]; then
 	while read x; do