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.

redo-sources.py

@@ -13,7 +13,5 @@ if len(sys.argv[1:]) != 0:
 
 cwd = os.getcwd()
 for f in state.files():
-    if f.name.startswith('//'):
-        continue  # special name, ignore
-    if not f.is_generated and f.read_stamp() != state.STAMP_MISSING:
+    if f.is_source():
         print state.relpath(os.path.join(vars.BASE, f.name), cwd)