revset: fix order of first/last members in compound expression (BC) Suppose len(subset) >> len(ls) in common cases, 'subset & ls' should be avoided whenever possible.

revset: filter first/last members by __and__ operation This replaces 'if y in subset' with '& subset'. first(null) and last(wdir()) are fixed thanks to fullreposet.__and__. This also revealed that first() and last() don't follow the order of the input set. 'ls & subset' is valid only if the ordering requirement is 'define' or 'any'. No performance regression observed: revset #0: limit(0:9999, 100, 9000) 0) 0.001164 1) 0.001135 revset #2: 9000 & limit(0:9999, 100, 9000) 0) 0.001224 1) 0.001181 revset #3: last(0:9999, 100) 0) 0.000237 1) 0.000199

revset: reject negative number to select first/last n members Negative 'lim' doesn't make sense here, and it makes things complicated when using list[:lim].