bug#27986: 26.0.50; `rename-file' can rename files without confirmation

[Philipp <p.stephani2@gmail.com>]

Run the following in *scratch*:

(make-directory "/tmp/emacs")
nil
(write-region "" nil "/tmp/emacs/ß")
nil
(write-region "" nil "/tmp/emacs/ẞ")
nil
(directory-files "/tmp/emacs")
("." ".." "ß" "ẞ")
(rename-file "/tmp/emacs/ẞ" "/tmp/emacs/ß")
nil

Note how `rename-file' has silently overwritten `ß'.  This is because on
macOS, `ß' and `ẞ' are different file names, but Emacs treats them as
equal.  Probably the test for case-insensitive file names should be
removed altogether (it can't work correctly and introduces a filesystem
race), and `rename-file' should use link(2) + unlink(2) if renameat2
isn't available.


In GNU Emacs 26.0.50 (build 77, x86_64-apple-darwin16.7.0, NS appkit-1504.83 Version 10.12.6 (Build 16G29))
 of 2017-08-06 built on p
Repository revision: b1b99edd3ee587a5154106d4d16547eac4916c55
Windowing system distributor 'Apple', version 10.3.1504
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --with-modules --without-xml2 --without-pop --with-mailutils
 --enable-gcc-warnings=yes MAKEINFO=/usr/local/opt/texinfo/bin/makeinfo
 'CFLAGS=-O3 -g0' LDFLAGS=-O3'

Configured features:
DBUS NOTIFY ACL GNUTLS ZLIB TOOLKIT_SCROLL_BARS NS MODULES

Important settings:
  value of $LANG: de_DE.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message subr-x puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils time-date tooltip eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel term/ns-win
ns-win ucs-normalize mule-util term/common-win tool-bar dnd fontset
image regexp-opt fringe tabulated-list replace newcomment text-mode
elisp-mode lisp-mode prog-mode register page menu-bar rfn-eshadow
isearch timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core term/tty-colors frame cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind kqueue cocoa ns multi-tty
make-network-process emacs)

Memory information:
((conses 16 204143 9065)
 (symbols 48 20146 1)
 (miscs 40 43 181)
 (strings 32 29176 1731)
 (string-bytes 1 784929)
 (vectors 16 35010)
 (vector-slots 8 710995 9656)
 (floats 8 48 68)
 (intervals 56 205 0)
 (buffers 992 11))