More fixes for converting missing targets -> sources.

I attempted to fix this in commit c06d1fba40, but it was apparently
incomplete and not all cases were covered by tests.

Let's add a much more thorough test by going through every possible
combination and making sure redo-{sources,targets,ood} all work as
expected, that the "you modified it" warning does or does not show up
when expected, and that dependencies are rebuilt the number of times we
expect.

t/351-deps-forget/.gitignore

@@ -1,6 +1,7 @@
 bork
-bork.do
 bork.log
 sub
 sub.log
-targets.out
+sub.warn
+silent.out
+want