Add "git diff" script

It's a simple helper that depending on the arguments will either
use git-diff-files, git-diff-cache or git-diff-tree.

Makefile

@@ -23,7 +23,7 @@ INSTALL=install
 SCRIPTS=git git-apply-patch-script git-merge-one-file-script git-prune-script \
 	git-pull-script git-tag-script git-resolve-script git-whatchanged \
 	git-deltafy-script git-fetch-script git-status-script git-commit-script \
-	git-log-script git-shortlog git-cvsimport-script
+	git-log-script git-shortlog git-cvsimport-script git-diff-script
 
 PROG=   git-update-cache git-diff-files git-init-db git-write-tree \
 	git-read-tree git-commit-tree git-cat-file git-fsck-cache \

git-diff-script

@@ -0,0 +1,16 @@
+#!/bin/sh
+rev=($(git-rev-parse --revs-only "$@"))
+flags=($(git-rev-parse --no-revs "$@"))
+case "${#rev[*]}" in
+0)
+	git-diff-files -p "$@";;
+1)
+	git-diff-cache -p "$@";;
+2)
+	begin=$(echo "${rev[1]}" | tr -d '^')
+	end="${rev[0]}"
+	git-diff-tree -p $flags $begin $end;;
+*)
+	echo "I don't understand"
+	exit 1;;
+esac