revset: build dict of extra sort options before evaluating set Prepares for extracting a function that only validates sort options.

revset: build list of (key, reverse) pairs before sorting Prepares for extracting a function that only validates sort options.

revset: fix crash on empty sort key Make it noop as before 2188f170f5b6. We could change it to an error, but allowing empty key makes some sense for scripting that builds a key string programmatically.