The bug here looks something like:

 * Compare files a, b and c
 * Modify b and c
 * Close the tab; when prompted to save, save both b and c
 * Both files will be saved, but the tab won't close

The problem here was that we were resetting the state to Normal when
a save succeeded. This doesn't cause an issue if you're only saving
one file, but if you're saving multiple then the second save to run
*won't* be in a Closing state in its saved callback, and so won't
try to close the tab.