Add minimal/do, a stripped-down redo implementation in 977 bytes of sh.

This could be good for distributing with your packages, so that people who
don't have redo installed can at least build it.  Also, we could use it for
building redo itself.

Will surely need to get slightly bigger as I inevitably discover I've
forgotten a critical feature.

t/bellow.do

@@ -1,3 +1,3 @@
 redo-ifchange LD yellow.o
-./LD bellow yellow.o
+./LD "$3" yellow.o
 ./sleep 2