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/differing-branches-simple

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+cd /tmp/demo
+echo '$ git switch master'
+git switch master
+echo '$ vim greeting.py'
+sed -i 's/Hello World/HELLO WORLD/g' greeting.py > /dev/null
+echo '  # CAPITALISE HELLO WORLD #'
+echo '$ git commit -am "Capitalises Hello World"'
+git -c color.ui=always commit -am "Capitalises Hello World"
+echo '$ git switch test'
+git switch test
+echo '$ vim greeting.py'
+sed -i '3iimport sys\n' greeting.py > /dev/null
+echo '  # Adds the line "import sys" #'
+echo '$ git commit -am "Adds sys import for arg parsing"'
+git commit -am "Adds sys import for arg parsing"
+#echo '$ git diff master..test'
+#git -c color.ui=always diff master..test
+#echo '$ git log --all --oneline --graph'
+#git -c color.ui=always log --all --oneline --graph --decorate=short