revset: avoid O(n) lookup of invalid revision in rev()
ba89f7b542c9 was not the final version of that patch. It was really slow
because `l not in repo.changelog` iterates revisions up to `l`. Instead,
rev() should utilize spanset.__contains__().
revset #0: rev(210000)
0) wall 0.000039 comb 0.000000 user 0.000000 sys 0.000000 (best of 67978)
1) wall 0.002721 comb 0.000000 user 0.000000 sys 0.000000 (best of 1055)
2) wall 0.000059 comb 0.000000 user 0.000000 sys 0.000000 (best of 45599)
(0: 3.2-rc, 1: ba89f7b542c9, 2: this patch)
Note that the benchmark result described in ba89f7b542c9 is wrong because
it is the one of the initial version.