Currently when checking if the destination branch already exists the
command 'git branch' is used. This only returns local branches. What is
actually required is the list of remote branches. This can be obtained
by the command 'git branch -r' and filtering the result.

Previously the script used
cmd =['git','checkout', branch]
syntax. This does not work well cross platform. The solution it to
actually use strings. E.g.
cmd = "git checkout " + branch