Suggestions

Hey @jni,

I just taught this tutorial and had some thoughts, but I didn't want to do a PR in case you complete disagree :stuck_out_tongue:

Firstly, I've decided I like it much better than the SWC version. It's much more succinct and has less side-tracks. It also focuses much more on branching and collaboration. Note, I taught a mini version because I only had 1.5 hours.

Suggestions:
- [ ] Replace all `git commit -a -m "message"` with:

```
git add mars.txt
git commit -m "message"
```

I know this is a common shortcut, but I think it's a bad habit, particularly for beginners. It leads to lots of accidental committing of things as the project gets more complicated.
- [ ] I also skipped the alias bit, because I think it should be a side-note/advanced feature, not part of a beginner tutorial.
- [ ] Finally, I did lots of graph pictures on the wall for the branching bit. Not sure if I have the patience to draw them electrically, but if I did, that would be my suggestion!