This fixes up some git and oscommand tests, and pulls some tests into commit_list_builder_test.go
I've also made the NewDummyBlah functions public so that I didn't need to duplicate them across packages
I've also given OSCommand a SetCommand() method for setting the command on the struct
I've also created a file utils.go in the test package for creating convient 'CommandSwapper's, which
basically enable you to assert a sequence of commands on the command line, and swap each one out for
a different one to actually be executed
