Adds references and branches slides

Mostly adds slides about references and branches. Also tweaked python
start code and re-aranged / tweaked some existing slides

shell/simple-merge

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+cd /tmp/demo
+echo '$ git switch master'
+git switch master
+echo '$ git merge test'
+git merge test
+echo '$ git log --oneline --all --graph'
+git -c color.ui=always log --oneline --all --graph --decorate=short