Fully grown pumpkin stems attach to pumpkin, even if another stem is already attached

The bug

If a pumpkin stem causes a pumpkin to grow on an adjacent block, all fully grown pumpkin stems that are not connected to a pumpkin already will connect to the new pumpkin and refuse to grow their own.

How to reproduce

1. On a 5x5 area, create a pumpkin farm similar to the following: http://i.imgur.com/C0WtPWP.png
2. Use bonemeal to make all four pumpkin stems fully grown.
3. Place a pumpkin in the center space.
4. Observe that all four pumpkin stems connect to it and will not spawn any additional pumpkins in the additional empty spaces regardless of how long you wait: http://i.imgur.com/2LY6pF4.png

Expected results

Pumpkin stems should not attach to any pumpkin that already has a stem attached to it.

The fix

A possible solution would be to use an additional bit in the pumpkin and melon's data values. This bit would act as a flag and activate as soon as one stem attaches to it, preventing other adjacent stems from also attaching to it.