* lisp/emacs-lisp/package.el: Don't erase tags on refresh

* lisp/emacs-lisp/package.el: Don't erase tags on refresh

(package-menu--post-refresh): Call `tabulated-list-print' with the
UPDATE argument.  This only affects the refresh action, the revert
action still erases tags.
(package-menu-get-status): Change `assq' to `assoc'.
(package-menu--mark-upgrades-1): New function.
(package-menu--mark-upgrades-pending): New variable.
(package-menu-mark-upgrades): Use them to delay marking until
after refresh is done.
(package-menu--post-refresh): Call mark-upgrades-1 if
mark-upgrades-pending is non-nil.