git: correct some signature mismatches between dirstate and the Protocol class These were flagged by PyCharm when subclassing the Protocol class. Note that both `is_changing_xxx` were only flagged when the Protocol class used a plain field, as mentioned in the previous commit. After converting those attrs in the Protocol class to @property to match the regular dirstate class, it stopped flagging these. But I don't think that makes sense- `@property` should look like an attribute to the outside world, not a callable.