repack: prune loose objects when -d is given

[jc: the request originally came from Martin Atukunda, which was
 improved further by Alex Riesen]
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-repack.sh

@@ -75,6 +75,7 @@ then
 		  done
 		)
 	fi
+	git-prune-packed
 fi
 
 case "$no_update_info" in