Thanks to a tip in this video by Cocoa Samurai/Colin Wheeler:
This is a screencast based off a talk I gave to the Des Moines Cocoaheads group. In this talk I tried to show why you should switch from Centralized Version Control, what is wrong with Subversion and what git does well. Check out Part II when I demonstrate Git and some apps available on Mac OS X to help you get the most out of git.
…you can configure File Merge to be used by default by Git version control system:
git config –global merge.tool opendiff
Haven’t tried it yet but it’s a great tip and I’m looking forward to implementing it once I get Git up and running properly…