misc/emacs: do not highlight built-in function if not followed by '('
Name of built-in function is not reserved word in Go, and you can
use it as variable name. "new" is often used as local variable, for
instance.
This patch is to apply font-lock-builtin-face only when built-in
function name is followed by '(', so that it doesn't highlight
non-function variable that happen to have the same name as built-in
function.
LGTM=dominik.honnef
R=golang-codereviews, dominik.honnef, adonovan
CC=golang-codereviews
https://golang.org/cl/
79260043