git-diffall

#!/bin/sh -e
# Copyright 2010, Tim Henigan <tim.henigan@gmail.com>
#
# Perform a directory diff between commits in the repository using
# the external diff tool specified in the 'diff.tool' configuration
# option.

USAGE='<options> <commit>{0,2} -- <path>*

--cached  Compare to the index rather than the working tree
commit    SHA1 of a commit
path      Limit the diff to the specified paths
'

. git-sh-setup

if [ -z $(git config --get diff.tool) ]; then
    echo "Error: The 'diff.tool' configuration option must be set."
    usage
fi

start_dir=$(pwd)
cd_to_toplevel      # needed to access tar utility

# mktemp is not available on all platforms (missing from msysgit)
# Use a hard-coded tmp dir if it is not available
if [ -z $(which mktemp) ]; then
    tmp=/tmp/git-diffall-tmp
else
    tmp="$(mktemp -d)"
fi
mkdir "$tmp" "$tmp"/a "$tmp"/b

left=
right=
paths=
path_sep=
compare_staged=
common_anscestor=

while test $# != 0; do
	case "$1" in
    -h|--h|--he|--hel|--help)
        usage
        ;;
    --cached)
        compare_staged=1
        ;;
    --)
        path_sep=1
        ;;
    -*)
        echo Invalid option: "$1"
        usage
        ;;
    *)
        # could be commit, commit range or path limiter
        case "$1" in
        *...*)
            left=${1%...*}
            right=${1#*...}
            common_anscestor=1
            ;;
        *..*)
            left=${1%..*}
            right=${1#*..}
            ;;
        *)
            if [ -n "$path_sep" ]; then
                if [ -z "$paths" ]; then
                    paths=$1
                else
                    paths="$paths $1"
                fi
            elif [ -z "$left" ]; then
                left=$1
            elif [ -z "$right" ]; then
                right=$1
            else
                if [ -z "$paths" ]; then
                    paths=$1
                else
                    paths="$paths $1"
                fi
            fi
            ;;
        esac
        ;;
    esac
    shift
done

# Determine the set of files which changed
if [ -n "$left" ] && [ -n "$right" ]; then
    if [ -n "$compare_staged" ]; then
        usage
    elif [ -n "$common_anscestor" ]; then
        git diff --name-only "$left"..."$right" -- "$paths" > "$tmp"/filelist
    else
        git diff --name-only "$left" "$right" -- "$paths" > "$tmp"/filelist
    fi
elif [ -n "$left" ]; then
    if [ -n "$compare_staged" ]; then
        git diff --name-only --cached "$left" -- "$paths" > "$tmp"/filelist
    else
        git diff --name-only "$left" -- "$paths" > "$tmp"/filelist
    fi
else
    if [ -n "$compare_staged" ]; then
        git diff --name-only --cached -- "$paths" > "$tmp"/filelist
    else
        git diff --name-only -- "$paths" > "$tmp"/filelist
    fi
fi

# Exit immediately if there are no diffs
if [ ! -s "$tmp"/filelist ]; then
    exit 0
fi

# Populate the tmp/b directory with the files to be compared
if [ -n "$right" ]; then
	while read name; do
		mkdir -p "$tmp"/b/"$(dirname "$name")"
		git show "$right":"$name" > "$tmp"/b/"$name"
	done < "$tmp"/filelist
elif [ -n "$compare_staged" ]; then
	while read name; do
		mkdir -p "$tmp"/b/"$(dirname "$name")"
		git show :"$name" > "$tmp"/b/"$name"
	done < "$tmp"/filelist
else
    tar -c -T "$tmp"/filelist | (cd "$tmp"/b && tar -x)
fi

# Populate the tmp/a directory with the files to be compared
while read name; do
	mkdir -p "$tmp"/a/"$(dirname "$name")"
    if [ -n "$left" ]; then
	    git show "$left":"$name" > "$tmp"/a/"$name"
    else
        if [ -n "$compare_staged" ]; then
            git show HEAD:"$name" > "$tmp"/a/"$name"
        else
            git show :"$name" > "$tmp"/a/"$name"
        fi
    fi
done < "$tmp"/filelist

cd "$tmp"
$(git config --get diff.tool) a b

# On exit, remove the tmp directory
cleanup () {
    cd "$start_dir"
	rm -rf "$tmp"
}

trap cleanup EXIT

This script is based on an example provided by Thomas Rast on the Git list [1]:
[1] http://thread.gmane.org/gmane.comp.version-control.git/124807