Turn off REWRITE_ARGS if `slacker update prio' is used, this
is certainly not what the user wants, as it rewrites PRIO according
to REPOPRIO.
Also remove a TODO: this does already work by this exact change:
if PRIO is properly set-up, i.e. patches comes first etc., then
doing `slacker update prio' will get all updates from patches
and the rest from series.