dagwalker: drop a useless intermediate variable The variable used to contain revs.set() but as the `.set()` has been removed it is now useless.

revset-node: speedup by a few hundred fold Instead of checking all elements of the subset against a single rev, just check if this rev is in the subset. The old way was inherited from when the subset was a list. Non surprise, this provide massive speedup. id("d82e2223f132") before) wall 0.008205 comb 0.000000 user 0.000000 sys 0.000000 (best of 302) after) wall 0.000069 comb 0.000000 user 0.000000 sys 0.000000 (best of 34518) revset #1: public() and id("d82e2223f132") before) wall 0.019763 comb 0.020000 user 0.020000 sys 0.000000 (best of 124) after) wall 0.000101 comb 0.000000 user 0.000000 sys 0.000000 (best of 20130)