bump(sys-apps/portage): sync with upstream

portage-2.2.8-r1 is stable, but keep the ebuild in the overlay disabled
for now.
feat(sys-apps/portage): Adding repos.conf patch to ebuild

Pulling in the patch mentioned here
(https://bugs.gentoo.org/show_bug.cgi?id=507284). This adds support for the
'disabled' attribute in repository sections within repos.conf.