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.