How can I highlight deleted and added lines like in this comment? How do I make it look like a git diff?
Do I have to use HTML? Or is there some tricky syntax like strikethrough?
That is done by using a code block with the "diff" syntax. The raw markdown would look something like this:
One small step for ```diff -man +humankind ```
I've made a gist also that demonstrates the code (click "Raw" to view the markdown).