revset: avoid in loop lookup in _generatorset._consumegen
Python lookups are slow, so do all lookup outside of the for loop.
This provide a small but still significant speedup:
revset #0: 0::
0) wall 0.063258 comb 0.060000 user 0.060000 sys 0.000000 (best of 100)
1) wall 0.057776 comb 0.050000 user 0.050000 sys 0.000000 (best of 100)