dirstate: rename `@requires_parents_change` to `@requires_changing_parents` This match the rename of the context manager.

dirstate: write dirstate on successful exit of changing_parents context This is the first step toward having more sensible and predicatable write patterns for the dirstate. Having better write/rollback patterns will greatly reduce and clarify the needs to backup the dirstate.

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.