revset-benchmark: add max(::(tip~20) - obsolete())

This revset is used in evolve. The new revset lazyness should make it all faster
but in practice it is significantly slower.

Below is a timing for this entry on my Mercurial repo.

2.9.2) ! wall 0.034598 comb 0.040000 user 0.040000 sys 0.000000 (best of 100)
3.0+@) ! wall 0.062268 comb 0.060000 user 0.060000 sys 0.000000 (best of 100)

The ~20 have been taken arbitrary.

contrib/revsetbenchmarks.txt

@@ -20,3 +20,4 b' public()'
 :10000 and public()
 draft()
 :10000 and draft()
+max(::(tip~20) - obsolete())