issue270_incremental_diff

ContentData for Date1 (noted as latest_snapshot):
```  
  Content1
     location1
     location2
```
Between Date1 and Date2 location2 was removed and no other file operations were done.

ContentData for Date2 (noted as content_data):
```  
  Content1
     location1
```
Then
content_data.remove_instances(latest_snapshot) is empty.