Araxis have changed their utility to 'compare2'. We assume 'compare'

As reported here: https://answers.atlassian.com/questions/92976/external-diff-command-won-t-launch-araxis-merge?page=1#comment-200863

This means newer versions of Araxis won't work from SourceTree. Neither can we assume that 'compare2' exists so need some kind of fallback in the event 'compare' isn't found (use a different script dynamically, perhaps?)