git-filemerge

#!/bin/sh
if test $# = 0; then
OLD=$(git write-tree)
elif test "$1" = --cached; then
OLD=HEAD
  NEW=$(git write-tree)
  shift
fi
if test $# -gt 0; then
OLD="$1"; shift
fi
test $# -gt 0 && test -z "$CACHED" && NEW="$1"
TMPDIR1=$(mktemp -d)
git archive --format=tar $OLD | (cd "$TMPDIR1"; tar xf -)
if test -z "$NEW"; then
TMPDIR2=$(git rev-parse --show-cdup)
test -z "$TMPDIR2" && TMPDIR2=.
else
TMPDIR2=$(mktemp -d)
  git archive --format=tar $NEW | (cd "$TMPDIR2"; tar xf -)
fi

opendiff "$TMPDIR1" "$TMPDIR2" | cat
rm -rf "$TMPDIR1"
test ! -z "$NEW" && rm -rf "$TMPDIR2"