bug#18790: 24.4; package-install no longer allows interactive updating

[Nic Ferrier]

M-x package-install [RET] existing-package [TAB]

used to allow you to upgrade a package without using the slightly
cumbersome package list.

The interactive form now filters the package list for installed
packages:

  (interactive
     (progn
       ;; Initialize the package system to get the list of package
       ;; symbols for completion.
       (unless package--initialized
         (package-initialize t))
       (unless package-archive-contents
         (package-refresh-contents))
       (list (intern (completing-read
                      "Install package: "
                      (delq nil
                            (mapcar (lambda (elt)
                                      (unless (package-installed-p (car elt))
                                        (symbol-name (car elt))))
                                    package-archive-contents))
                      nil t)))))

There isn't a simple equivalent of doing the same thing, say:

  M-x package-upgrade-existing-package

I don't know if this was deliberate or a mistake, it's always useful
being able to do something quickly with a command line when you know
what you're doing.