Add cyclic dependence detection.

If a depends on b which depends on a, redo would just freeze.  Now it
aborts with a somewhat helpful error message.

[Updated by apenwarr for coding style and to add a test.]

vars_init.py

@@ -34,3 +34,5 @@ def init(targets):
 
         import state
         state.init()
+
+    os.environ['REDO_LOCKS'] = os.environ.get('REDO_LOCKS', '')