Don't syntax highlight diffs

[Graham42]

The extra color is more confusing. Syntax highlighting is great for writing code, where the focus is on keywords. However, for diffs the focus should be on what changed. Syntax highlighting for diffs confuses that.

Reference: https://twitter.com/keplersj/status/933591387903164416

[felixfbecker]

I think the syntax highlighting is good, but just not well implemented. It mixes the keyword highlighting with the diff highlighting:

I think it would be better if not-changed lines had syntax highlighting, but +/- lines were all green and red, respectively. Or the syntax highlighting could always be there, but the +/- lines have green/red background color.