bug#41412: 27.0.90; Value of default directory in completions buffer

[Karthik Chikmagalur <karthikchikmagalur@gmail.com>]

The value of `default-directory' in the completions buffer is wrong when
there is a partial completion inserted in the minibuffer.

Steps to reproduce:

1. Start `emacs -Q'
2. C-x C-f to start finding a file
3. Enter a string (at least one character) and hit tab to pop up the
completions buffer. Let's say this string was "test"
4. The completions buffer will have completions starting with
"test". (We need at least one candidate).
5. C-h v default-directory
6. The value of `default-directory' in the completions buffer is set to
"~/test/", or the current directory + whatever string the user entered
into the minibuffer.


In GNU Emacs 27.0.90 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.13, cairo version 1.17.3)
 of 2020-03-31 built on thinkpad
Repository revision: ac3da1dd9626bc475e78b18c0b438435790e8926
Repository branch: makepkg
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Manjaro Linux

Recent messages:
Making completion list...
Quit [2 times]
Making completion list... [2 times]

Quit
You can run the command ‘load-theme’ with M-x lo-t RET
Use +,-,0 for further adjustment [2 times]
Making completion list...
Type "q" in help window to delete it.
Quit [3 times]
Quit
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
 --with-sound=alsa --with-modules --without-gconf --without-gsettings
 --with-x-toolkit=gtk3 --without-xaw3d --without-m17n-flt --with-cairo
 --without-compress-install 'CFLAGS=-march=x86-64 -mtune=generic -O2
 -pipe -fstack-protector-strong -fno-plt -g -flto -g -flto -s
 -fuse-ld=gold' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GLIB NOTIFY INOTIFY ACL
LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD JSON
PDUMPER LCMS2 GMP

Important settings:
  value of $LC_MONETARY: en_US.UTF-8
  value of $LC_NUMERIC: en_US.UTF-8
  value of $LC_TIME: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  shell-dirtrack-mode: t
  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 rmc puny dired dired-loaddefs
format-spec rfc822 mml mml-sec password-cache epa derived epg epg-config
gnus-util rmail rmail-loaddefs text-property-search mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils face-remap
dichromacy-theme time-date misearch multi-isearch vc-git diff-mode
easy-mmode cl-extra seq byte-opt gv bytecomp byte-compile cconv pp
cl-print mode-local find-func thingatpt help-fns radix-tree help-mode
easymenu shell pcomplete comint ansi-color ring dabbrev embark edmacro
kmacro cl-loaddefs cl-lib pcase subr-x tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer 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 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 threads dbusbind inotify lcms2 dynamic-setting
font-render-setting cairo move-toolbar gtk x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 68828 9827)
 (symbols 48 7742 1)
 (strings 32 23173 2123)
 (string-bytes 1 702281)
 (vectors 16 12996)
 (vector-slots 8 159649 11786)
 (floats 8 46 26)
 (intervals 56 330 0)
 (buffers 1000 17))