dirstate: rename parentchange to changing_parents Since the new argument breaks the API anyway, we can rename it to a better name. The previous name `parentchange` might be seen as something active, a function that would directly change the parents, however this is just a context manager to frame the operation that will change the parents and adjust the dirstate content accordingly. In addition, the future sister method that will be about changes to tracking and files would have a hard time fitting in the same naming scheme in a clear way. The new naming uses a clear prefix will make it more distinct from other dirstate methods and easier to extend with other similar contexts.

dirstate: pass the repo to the `changeparent` method If we want the context to be responsible for writing (and we want it), we need to have access to a localrepository object. So we now requires a localrepository object as an argument to this context manager.

dirstate: warn about non-explicitly rolledback parent-change Now that the invalidation is dealt with by the context manager itself, we should no longer need this in the unlocking code. We start with issuing a warning if this is the case before actually dropping this code later.