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.py

@@ -28,3 +28,14 @@ os.environ['REDO_UNLOCKED'] = ''  # not inheritable by subprocesses
 
 NO_OOB = os.environ.get('REDO_NO_OOB', '') and 1 or 0
 os.environ['REDO_NO_OOB'] = ''    # not inheritable by subprocesses
+
+
+def get_locks():
+  """Get the list of held locks."""
+  return os.environ.get('REDO_LOCKS', '').split(':')
+
+def add_lock(name):
+  """Add a lock to the list of held locks."""
+  locks = set(get_locks())
+  locks.add(name)
+  os.environ['REDO_LOCKS'] = ':'.join(list(locks))