html/template: clean up locking for ExecuteTemplate
html/template: clean up locking for ExecuteTemplate

R=mikesamuel, rogpeppe
CC=golang-dev
http://codereview.appspot.com/5448137

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang