@glenn already got the answer, but I’ve found a simpler way of showing the git prompt on fish.
From the terminal, in fish, type fish_config. This will open a browser window. Select the second tab prompt and under there select Classic + Git`.
This will show the commands required to show Git on the terminal prompt. Copy them to your ~/.config/fish/config.fish or even simpler: click on “Use prompt”.
How awesome is that?