bug#31305: 27.0.50; Symlinks recognized as dirs

[Jimmy Yuen Ho Wong]

Reproduction:

0. Get a Mac
1. Download the most recent Emacs 26 nightly from 
https://emacsformacosx.com/builds
2. Symlink a file in a dir to another file in another dir
3. C-x C-f some prefix to the symlink
4. TAB for completion
5. Completion completed the path with an extra /
6. RET
7. Emacs opens up the dir as a new file in a fundamental mode buffer

Expectation:
Completion should not complete the symlink as a dir, opening the dir as
a regular file instead of dired should also not be possible.


In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin13.4.0, NS 
appkit-1265.21 Version 10.9.5 (Build 13F1911))
of 2018-04-18 built on builder10-9.porkrind.org
Windowing system distributor 'Apple', version 10.3.1561
System Description: Mac OS X 10.13.4

Recent messages:
Warning: arch-dependent data dir 
’/Users/build/workspace/Emacs-Multi-Build/label/mavericks/emacs-source/nextstep/Emacs.app/Contents/MacOS/libexec/’: 
No such file or directory

For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
'configure --with-ns '--enable-locallisppath=/Library/Application
Support/Emacs/${version}/site-lisp:/Library/Application
Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES THREADS

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

Major mode: Fundamental

Minor modes in effect:
tooltip-mode: t
global-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
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
buffer-read-only: t
line-number-mode: t
transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv 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 cl-loaddefs cl-lib sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils time-date elec-pair
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 kqueue cocoa ns
multi-tty make-network-process emacs)

Memory information:
((conses 16 204043 8971)
(symbols 48 19821 1)
(miscs 40 69 175)
(strings 32 28749 2003)
(string-bytes 1 763036)
(vectors 16 35111)
(vector-slots 8 715296 15296)
(floats 8 47 69)
(intervals 56 206 0)
(buffers 992 12))