This patch makes sure that if you translate/scale something in editmode with mirror clipping, it will stay on it's constraint axis/plane even if that doesn't align with the clipping plane normal. This is especially useful when using multiple rotated mirror modifiers.
The behaviour for non constraint translation/scaling is unchanged.
Here is one very useful example of the new behaviour:
Here is one example where the clipping behaviour is changed. The already clipped vertices will stay in place in this case.