bug#53604: 29.0.50; read-buffer-to-switch unconditonally overrides minibuffer-completion-table

[Visuwesh]

`read-buffer-to-switch', used by `switch-to-buffer', unconditionally
sets `minibuffer-completion-table' to `rbts-completion-table'.  It
should only do that when `read-buffer-function' is set to nil since
`read-buffer-function' cannot do its job properly.

As a minimal reproduction, consider the following [a]

    1. emacs -Q
    2. Evaluate the following form,
        (let ((read-buffer-function
               (lambda (&rest _)
                 (completing-read "prompt: " (list "1" "2")))))
          (call-interactively #'switch-to-buffer))
    3. Type TAB.
    4. *Completions* buffer does not show "1" "2".  If you replace
       (call-interactively ...) with (read-buffer ""), it does.

a. To justify my request, I will write my problem here.  I wrote a
custom read-buffer-function to include "virtual" buffer candidates
(popularised by ivy, helm, etc.) like recentf files, bookmarks, etc.,
but the current behaviour makes my job much harder (or uglier i.e.,
advice) as I have to write custom switch-to-buffer-* commands and remap
the standard switch-to-buffer-*.



BTW, should the docstring of `switch-to-buffer' mention
`read-buffer-to-switch'?  I see that Lars decided against it in bug#23131
(https://lists.gnu.org/archive/html/bug-gnu-emacs/2021-07/msg00703.html).

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 
1.16.0, Xaw scroll bars)
Repository revision: 4b7e73432cd07612e5225189e18696f755c8f954
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: NixOS 21.11 (Porcupine)

Configured using:
 'configure
 --prefix=/nix/store/rsgfliii3k06j8i77zn295yp87qbq0yr-emacs-git-20211221.0
 --disable-build-details --with-modules --with-x-toolkit=lucid
 --with-xft --with-cairo --with-native-compilation'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NATIVE_COMP NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM LUCID ZLIB

Important settings:
  value of $EMACSLOADPATH: 
  value of $EMACSNATIVELOADPATH: 
/nix/store/x6fjznfj7wks7nvs6xgsk1q4fcd5rjrw-emacs-packages-deps/share/emacs/native-lisp::
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Help

Minor modes in effect:
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semantic-show-unmatched-syntax-mode)  S)))
  shell-dirtrack-mode: t
  recentf-mode: t
  eros-mode: t
  pdf-occur-global-minor-mode: t
  minibuffer-depth-indicate-mode: t
  repeat-mode: t
  display-time-mode: t
  display-battery-mode: t
  straight-use-package-mode: t
  straight-package-neutering-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tab-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  indent-tabs-mode: t
  transient-mark-mode: t

Load-path shadows:
/home/viz/.nix-profile/share/emacs/site-lisp/site-start hides 
/nix/store/x6fjznfj7wks7nvs6xgsk1q4fcd5rjrw-emacs-packages-deps/share/emacs/site-lisp/site-start
/home/viz/lib/emacs/straight/build/map/map hides 
/nix/store/rsgfliii3k06j8i77zn295yp87qbq0yr-emacs-git-20211221.0/share/emacs/29.0.50/lisp/emacs-lisp/map
/home/viz/lib/emacs/straight/build/let-alist/let-alist hides 
/nix/store/rsgfliii3k06j8i77zn295yp87qbq0yr-emacs-git-20211221.0/share/emacs/29.0.50/lisp/emacs-lisp/let-alist
/nix/store/rsgfliii3k06j8i77zn295yp87qbq0yr-emacs-git-20211221.0/share/emacs/29.0.50/lisp/emacs-lisp/eieio-compat
 hides 
/nix/store/rsgfliii3k06j8i77zn295yp87qbq0yr-emacs-git-20211221.0/share/emacs/29.0.50/lisp/obsolete/eieio-compat

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tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
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rfn-eshadow isearch easymenu timer select scroll-bar mouse jit-lock
font-lock syntax font-core term/tty-colors frame minibuffer cl-generic
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Memory information:
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