When fetching alternates, http-fetch may reuse the slot to fetch non-http
alternates if http-alternates does not exist. When doing so, it now needs
to update the slot's finished status so run_active_slot waits for the
non-http alternates request to finish.
Signed-off-by: Nick Hengeveld <nickh@reactrix.com> Signed-off-by: Junio C Hamano <junkio@cox.net>