bug#14340: 24.3.50; Case insensitivity with read-file-name

[Harald Hanche-Olsen <hanche@math.ntnu.no>]

This concerns what seems like wrong handling of a buffer local
variable which is let bound. To reproduce:

From emacs -Q, evaluate the following in the *scratch* buffer,
one by one:

(setq read-file-name-completion-ignore-case t)
(read-file-name "file: ")
;;; confirm that filename completion is case insensitive
(setq-local completion-ignore-case t)
(read-file-name "file: ")
;;; notice that filename completion is case sensitive,
;;; contrary to expectation

My configuration is below (emacs compiled from trunk on the day
indicated, abot three weeks ago); however, note that on the
emacs-devel list, Michael Heerdegen reports that he could reproduce
this on GNU/Linux.

In GNU Emacs 24.3.50.1 (x86_64-apple-darwin12.3.0, NS apple-appkit-1187.37)
 of 2013-04-11 on airy
Windowing system distributor `Apple', version 10.3.1187
Configured using:
 `configure --with-ns'

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

- Harald