Correctly handle a checksummed file that depends on a non-checksummed file.

We were rebuilding the checksummed file every time because redo-ifchange was
incorrectly assuming that a child's changed_runid that's greater than my
changed_runid means I'm dirty.  But if my checked_runid is >= the child's
checked_runid, then I'm clean, because my checksum didn't change.

Clear as mud?

redo-ifchange.py

@@ -38,7 +38,7 @@ def dirty_deps(f, depth, max_changed):
                 return True
         elif mode == 'm':
             if dirty_deps(f2, depth = depth + '  ',
-                          max_changed = f.changed_runid):
+                          max_changed = max(f.changed_runid, f.checked_runid)):
                 debug('%s-- DIRTY (sub)\n' % depth)
                 return True
     if f.is_override: