exp/template/html: check that modified nodes are not shared by templates
exp/template/html: check that modified nodes are not shared by templates

R=nigeltao
CC=golang-dev
http://codereview.appspot.com/5012044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go