misc/emacs: ignore backquote in comment or string

misc/emacs: ignore backquote in comment or string

go-mode on Emacs 23 wrongly recognizes a backquote in a comment or
a string as a start of a raw string literal. Below is an example
that go-mode does not work well. This patch is to fix that issue.

  // `
  var x = 1
  // `

LGTM=dominik.honnef
R=golang-codereviews, dominik.honnef, adonovan
CC=golang-codereviews
https://golang.org/cl/84900043