On old macintoshes, there was an app named "FileMerge.app" that was buried deep inside the XCode executable. This was The Perfect GUI diff tool as far as I can tell. I could run it on files. I could run it on directories. I could have hg or git run it on diffs and let me inspect patches before I committed them, I could have it automatically run on merge. On every computer I have used post Mac OS 10.6 I have lacked this program and its absence is like having a limb gone
