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.

TODO

@@ -3,7 +3,11 @@
 	jwack jobserver
 		make compatibility
 		sockety interface
-	sqlite .redo database
+	sqlite .redo database?
 	rename --ifchange/--ifcreate to separate programs
 	rewrite ifchange/ifcreate in C
-	
+	always update database atomically?
+	find the right .repo dir automatically
+	cd into subdir automatically
+	'redo clean' should work
+	aggressively mark stuff as done in the .redo database

clean.do

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

t/bellow.do

@@ -0,0 +1,2 @@
+redo --ifchange LD yellow.o
+./LD bellow yellow.o

t/it.do

@@ -1 +1,2 @@
-redo --ifchange hello
+redo --ifchange hello yellow bellow
+

t/yellow.do

@@ -0,0 +1,2 @@
+redo --ifchange LD yellow.o
+./LD yellow yellow.o

t/yellow.o.do

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