bug#23407: .dir-local settings get obliterated on running a major mode function.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

In the master branch (I'm sure it's the same in the emacs-25 branch,
too):
1. start Emacs with emacs -Q
2. Visit any emacs lisp file which is part of Emacs with C-x C-f.
3. Do C-h C-v indent-tabs-mode <RET>.  The *Help* buffer starts off
  with:

    indent-tabs-mode is a variable defined in `C source code'.
    Its value is nil
    Original value was t
    Local in buffer follow.el; global value is t

  .  The local value comes from the .dir-locals in the top level Emacs
  directory.
4. Do M-x emacs-lisp-mode.
5. Again do C-h C-v indent-tabs-mode <RET>.  This time, the *Help*
  buffer starts off with:

    indent-tabs-mode is a variable defined in `C source code'.
    Its value is t

  .  This is different from the first *Help* buffer.
6. This is a bug.  On running a major mode, directory local settings
  should be respected.

-- 
Alan Mackenzie (Nuremberg, Germany).