text/template/parse, html/template: copy Tree.text during html template clone

The root cause of the panic reported in https://code.google.com/p/go/issues/detail?id=5980
is that parse's Tree.Text wasn't being copied during the clone.

Fix this by adding and using a Copy method for parse.Tree.

Fixes issue 5980.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/12420044

Committer: Rob Pike <r@golang.org>