Update GitCheatSheet to align with workflow

This commit restructures GitCheatSheet.txt to align with the workflow we're all supposed to
be following.. In particular, the commands you're likely to want to run are different when
working with a feature branch, so this way of organizing it reflects that.

This is mostly just reordering the instructions in the file to align with the workflow at the top.
Can this be written without the `../util` prefix? If using the `setchplenv.*` scripts, start_test will be on the PATH.