git_diff.rb

#!/usr/bin/env ruby

require 'tempfile'
require 'rubygems'
require 'mojombo-grit'

include Grit

PATH_TO_DIFFMERGE = "/usr/bin/diffmerge"

repo = Repo.new('.')
head = repo.commits.first
tree = head.tree

blob = tree/ARGV[0]

tempfile = Tempfile.new(blob.name)
tempfile.puts blob.data
tempfile.flush

pid = fork
if pid # parent
  Process.wait(pid)
else    # child
  exec("#{PATH_TO_DIFFMERGE} #{tempfile.path} #{ARGV[0]}")
end
tempfile.close