Pressing "Done" in a command block without changing command resets its SuccessCount

The bug

Pressing the "Done" button in the command block GUI resets the success count even when the command was not changed.

How to reproduce

1. Place a command block
2. Enter for example the following command

   /testfor @p
   

3. Run it once
4. Look at its block data. It will have a SuccessCount of non-zero.
5. Open the command block, change some settings if you want, hit "Done"
6. Look at the block data again. It will now have a SuccessCount of 0, even though the command did not change.