Sprite folders addon breaks diffing

[ajskateboarder]

Enabling the sprite folders addon causes a couple of bugs:

• if the user has a sprite open that is not labelled as selected in the sprite pane, scratch.git will think you are in the stage
• sprites are named [folderName]//[sprite], but since scratch.git uses the name of the selected sprite which is usually just [sprite], it will report that nothing was changed on the frontend

[ajskateboarder]

I'm pretty sure users can see changes now, only that when they move sprites in/out of a folder, the previous version won't show outside/inside the folder. Not a big deal