git_diff.rb

#!/usr/bin/env ruby

PATH_TO_DIFFMERGE = "/Applications/DiffMerge.app"

at_exit {
  call("rm -rf clone/")
}

def call(cmd)
  IO.popen(cmd) {|io| io.read }
end

files = call("git diff --cached --name-only").split
fail "No changes." if (files.empty?)

puts "Be patient..."
call("rm -rf clone/")
call("git clone . clone; cd clone/; git reset --hard HEAD")

loop {
  puts "Changed files are:"
  files.each_with_index{|file, i| puts "\t#{(i+1)}:\t#{file}" }
  print "Which shall we diff?:"
  num = gets.to_i
  if (num == 0 || num > files.length)
    puts "Uhhh no, I didn't offer #{num}..."
  else
    file = files[num-1]
    pwd = call("pwd").strip
    IO.popen("cd #{PATH_TO_DIFFMERGE}/Contents/MacOs/; ./DiffMerge #{pwd}/clone/#{file} #{pwd}/#{file}")
  end
}