Complicate the test build a bit to show unnecessary dependency re-checking.

'./redo -d test' reveals that we check and re-check things like CC and LD
even when they're children of objects that don't need to be rebuilt, and who
we've already checked earlier in the build cycle.  We need to be a little
smarter about pruning if we want dependency checking to be fast.

t/yellow.o.do

@@ -0,0 +1,2 @@
+redo --ifchange CC hello.c
+gcc -o $3 -c hello.c