If the created target is a directory, it's okay for the .do to create it.

Normally, creating the target $1 yourself is bad; create $3 instead.  But if
$1 is a directory, we'll allow it.  That way 'redo subdir' can call
subdir.do, and subdir.do can both create the directory *and* run a bunch of
sub-.do files on it.

t/test.do

@@ -1,4 +1,4 @@
 redo-ifchange all
 ./hello >&2
 redo deltest deltest2 test.args test2.args passfailtest \
-	curse/test deps/test "space dir/test" modetest
+	curse/test deps/test "space dir/test" modetest makedir