revset: make internal _list() expression remove duplicated revisions This allows us to optimize chained 'or' operations to _list() expression. Unlike _intlist() or _hexlist(), it's difficult to remove duplicates by the caller of _list() because different symbols can point to the same revision. If the caller knows all symbols are unique, that probably means revisions or nodes are known, therefore, _intlist() or _hexlist() should be used instead. So, it makes sense to check duplicates by _list() function. '%ls' is no longer used in core, this won't cause performance regression.