html/template: update the Tree field after parsing new templates
After text/template.Parse, all the templates may have changed, so
we need to set them all back to their unescaped state. The code
did this but (mea culpa) forgot to set the Tree field of the html/template
struct.

Since the Tree is reset during escaping, this only matters if an error
arises during escaping and we want to print a message.

Fixes issue 6459.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/13877043