This test not only tests the correct handling and display of the updateRef
command, but also the visualization of branch heads in the commits panel. Since
we are about to change the behavior here, extend the test so that a master
commit is added (we don't want this to be visualized as a branch head), and then
a stack of two non-main branches. At the end of this branch we only want to
visualize the head commit of the first.
cb240081a8