Editor on git index is no longer read-only

- Edit a file tracked by Git
- Stage the file
- In the SCM viewlet, click the file in the "staged" section to open an editor that says `(Index)`
- Type in the file
- In 1.24.1, I can't edit it because it's a read-only editor
- In 1.25, I can type in the editor, but changes aren't actually written anywhere. If I switch away from the file and switch back, they are gone