> piston not cutting the redstone dust power while the dust is being cut visually

Steps to reproduce:

1. Place a redstone wire so that it rises up 1 block as it turns 90 degrees (see piston-cut.png).
2. Power the wire at the lower end.
3. Place a piston above the lower wire so that it visually cuts the wire.

Expected results:
Either the piston cuts the wire or it doesn't. If it cuts the wire, there is no connection between the lower and upper parts of the wire, so no power source for the piston and it will retract. If it doesn't cut the wire, the wire will visually climb the side of the block, the wire on the upper block will be powered, and the piston will extend.

Actual results:
The upper wire is powered and powers the piston, which is extended, so it looks like the piston doesn't cut the wire. However, the wire doesn't visually climb the block, so it looks like the piston does cut the wire.