Based on this article, I modified it a little bit in order to make it work on Mac:

[diff]   tool=diffmerge                                                                                                                   
[difftool "diffmerge"]                                                                                                                      
  cmd = exec /Applications/DiffMerge.app/Contents/MacOS/DiffMerge \"$LOCAL\" \"$REMOTE\"                                                                                                                                   
[merge]                                                                                                                                     
  tool = diffmerge                                                                                                                    
[mergetool "diffmerge"]                                                                                                                            
  cmd = exec /Applications/DiffMerge.app/Contents/MacOS/DiffMerge --merge --result=\"$MERGED\"\n\"$LOCAL\" \"$BASE\" \"$REMOTE\"                                                                                           
  trustexitcode = false