dirstate: factor the identity getting/setting code in the dirstate map We are doing the same things twice and we will add more logic in the next changesets. So lets start factoring things out now.

dirstate: use `cachestat` object for dirstatemap identity There is a class dedicated to this kind of cache check, let us use it. We will generalize this code in the next changesets, but we do the "behavior changing" pass on our own.

automv: lock the repository before searching for renames I detected this while debugging something else.

dirstate: distinct transaction callback from largefile This has not caused any issue so far because the large-files dirstate bypass the transaction logic. We might want to change that. In anyway, let us disarm this bug nest before it actually explode.

dirstate: track that changes are pending in a transaction Nothing is currently broken because if this, but this make the `_invalidated_context` attribute more accurate. Being more accurate here will help us later, when dealing with `status` call.

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.

dirstate: use the new `check_invalidated` decorator for `_changing` WeeeEEeee, less code.

dirstate: introduce a check_invalidated decorator This is a common need, so let us consolidate it to simplify the code.

dirstate: warn if dirty when starting an edition The dirstate should be clean before we start changing it. Otherwise we might write unrelated changes. Having a dirty dirstate laying around is also suspicious. This is similar to what we do when opening a new transaction, but this time this affect dirstate changes outside of a transaction.