dirstate: rename `pendingparentchange` to `is_changing_parents` This is clearer and more inline witht he other change we did.

dirstate: rename _parentwriters to _changing_level We will have context for changing more than parents, so lets be prepared.

largefiles: remove the `changing_parents` context in `openlfdirstate` This code might run in any kind of change context, we should rely on the higher level context instead of opening our own.