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