bug#41436: 26.1; How to list upgradable packages only

[Eduardo Mercovich <eduardo@mercovich.net>]

Dear Emacs Devs.

Along with my thanks, I'd like to report a feature request, as per the
discussion in the Emacs Reddit group
(https://www.reddit.com/r/emacs/comments/gnci5o/how_to_list_upgradable_packages_only/)
and the suggestion from https://www.reddit.com/user/cpitclaudel/

Here is the case: 

--8<---------------cut here---------------start------------->8---

Upon refreshing the repos, the emacs package manager (PM) tells us
if/how many packages we have to upgrade. With U we can mark them all,
and with X let the PM do it's job (uninstall old versions, install the
new ones). Up to here, everything is great. 

However, being a rookie, I' afraid of upgrading 28 packages at once
since some things may break, so I prefer to upgrade them in small groups
in a more controlled way. 

But, for my life, even after quite some searching and reading I can't
find how to filter the list by that status (the command
package-menu-filter does something different or I don't understand how
to use it for this), so I can see only the packages that can be
upgraded. With that list in front of me, I can pick which ones to
upgrade, slowly and in a more controlled path. 

I'm sure something like that exists, but obviously I'm not finding the
right keywords.

--8<---------------cut here---------------end--------------->8---

Again, thanks a lot for sharing your work with all of us. :)

What follows is part of my local install, although it's probably useless
in this case.

In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.5)
 of 2019-09-23, modified by Debian built on x86-grnet-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description:	Debian GNU/Linux 10 (buster)

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --build
 x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
 --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs-StqULU/emacs-26.1+1=.
 -fstack-protector-strong -Wformat -Werror=format-security -Wall'
 'CPPFLAGS=-Wdate-time -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'
Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 THREADS LIBSYSTEMD LCMS2
Important settings:
  value of $LANG: es_AR.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Org

Minor modes in effect:
  diff-auto-refine-mode: t
  shell-dirtrack-mode: t
  auto-image-file-mode: t
  flyspell-mode: t
  org-indent-mode: t
  org-roam-mode: t
  display-time-mode: t
  desktop-save-mode: t
  pdf-occur-global-minor-mode: t
  wrap-region-mode: t
  async-bytecomp-package-mode: t
  helm-mode: t
  global-hi-lock-mode: t
  hi-lock-mode: t
  electric-pair-mode: t
  delete-selection-mode: t
  savehist-mode: t
  smartparens-global-mode: t
  smartparens-mode: t
  org-bullets-mode: t
  beacon-mode: t
  override-global-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-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
  line-number-mode: t
  global-visual-line-mode: t
  visual-line-mode: t
  transient-mark-mode: t
  abbrev-mode: t

Memory information:
((conses 16 2194798 1708575)
 (symbols 48 68434 188)
 (miscs 40 1962 3428)
 (strings 32 296075 153673)
 (string-bytes 1 9977187)
 (vectors 16 84246)
 (vector-slots 8 2071025 227004)
 (floats 8 1245 490)
 (intervals 56 271031 239055)
 (buffers 992 38))

-- 
eduardo mercovich

 Donde se cruzan tus talentos 
 con las necesidades del mundo, 
 ahí está tu vocación. 
 (Anónimo)