Improved error message from git-rebase

If the index wasn't clean, git-rebase would simply show the output from
git-diff-index with no further comment to the user.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-rebase.sh

@@ -249,7 +249,8 @@ fi
 git-update-index --refresh || exit
 diff=$(git-diff-index --cached --name-status -r HEAD)
 case "$diff" in
-?*)	echo "$diff"
+?*)	echo "cannot rebase: your index is not up-to-date"
+	echo "$diff"
 	exit 1
 	;;
 esac