Use normal double quotes in TeX / LaTeX comments

* lisp/textmodes/tex-mode.el (tex-insert-quote): When in a comment,
  default to inserting a normal double quote (") instead of TeX-style
  open (``) or close ('') quote marks.

* etc/NEWS (TeX mode): Note the change.

* test/lisp/electric-tests.el (autowrapping-7): Adjust accordingly.

See this thread for discussion:

  https://lists.gnu.org/archive/html/emacs-devel/2016-08/msg00611.html
  From: Karl Fogel <kfogel {_AT_} red-bean.com>
  To: Emacs Development <emacs-devel {_AT_} gnu.org>
  Subject: [PATCH] Have LaTeX mode use normal double quotes in comments.
  Date: Mon, 29 Aug 2016 14:44:12 -0500
  Message-ID: <87twe3icmr.fsf@red-bean.com>

etc/NEWS

@@ -1477,6 +1477,9 @@ looking for macro definitions.  By default, no symbols are ignored.
 
 ** TeX mode
 
+*** When in a TeX (LaTeX, etc) comment, insert a normal double quote (")
+instead of defaulting to TeX-style open (``) or close ('') quote marks.
+
 *** New custom variable 'tex-print-file-extension' to help users who
 use PDF instead of DVI.
 

lisp/textmodes/tex-mode.el

@@ -1311,6 +1311,7 @@ inserts \" characters."
   ;;
   (if (or arg (memq (char-syntax (preceding-char)) '(?/ ?\\))
           (eq (get-text-property (point) 'face) 'tex-verbatim)
+          (nth 4 (syntax-ppss)) ; non-nil if point is in a TeX comment
           ;; Discover if a preceding occurrence of `tex-open-quote'
           ;; should be morphed to a normal double quote.
           ;;

test/lisp/electric-tests.el

@@ -578,6 +578,7 @@ baz\"\""
 (define-electric-pair-test autowrapping-7
   "foo" "\"" :expected-string "``foo''" :expected-point 8
   :modes '(tex-mode)
+  :test-in-comments nil
   :fixture-fn #'(lambda ()
                   (electric-pair-mode 1)
                   (goto-char (point-max))