I believe this is how svn diff operates... but anyway, this alias will allow you to diff the last two commits in Git through TextMate. If you have a theme that supports Diff's then you'll see the differences clearly. Remove the last pipe statement (the mate part) to expose your diff in the console.
alias gitdiff='git log|grep commit|cut -d " " -f2|head -n 2|xargs -n 2 git diff -R|mate'
