Changes: prevent going to the previous or next file when the compare was forced

In other words, if you:

- Select a big file
- Click the "Force Compare" button
- Use the "Go to previous change" or "Go to next change" buttons until you reach the start or end of the file

it would be nice if SmartGit wouldn't select the previous or the next file. Because then you have to wait again for the compare to be computed for the big file.