dirstate: add small asserts for double security We don't need this, but it does not hurt.

dirstate: simplify the invalidation management on context exit Since the `invalidate` call will directly reset the `_invalidated_context` attribut, we can simplify the code. In the same go, we move most of the logic out of the `finally` clause. It seems cleaner and safer. If we are handling an exception, we don't need the `write` code anyway.