Add more demo gifs

[jesseduffield]

Is your feature request related to a problem? Please describe. We need more gifs in the readme to capture more of our features:

• [x] branch graph, show highlighting as you go
• [x] diff branches
• [x] rebase --onto
• [ ] compare branch with upstream (symmetric difference)

I'm pretty sure this covers the important stuff.

[jesseduffield]

test comment for discord server