11: Use dissimilar for diffs instead of difference r=Jesse-Bakker a=Jesse-Bakker

As discussed with @matklad in https://github.com/rust-analyzer/rust-analyzer/pull/7106.

Before:
![expect-test-screen-difference](https://user-images.githubusercontent.com/22473248/103417757-4d573e80-4b8c-11eb-8f5e-3f0597743279.png)

After:
![expect-test-screen](https://user-images.githubusercontent.com/22473248/103417761-4defd500-4b8c-11eb-9497-33d65ae60e07.png)

Used background instead of foreground colours because I think the diff boundaries are clearer, but I'm not married to that change ;)

Co-authored-by: Jesse Bakker <github@jessebakker.com>