revset: fix O(2^n) perf regression in addset

hg log -r 1 ... -r 100 was never returning due to a regression in the
way addset computes __nonzero__. It used 'bool(self._r1 or self._r2)'
which required executing self._r1.__nonzero__ twice (once for the or,
once for the bool). hg log with a lot of -r's happens to build a one
sided addset tree of N length, which ends up being 2^N performance.

This patch fixes it by converting to bool before or'ing.

This problem can be repro'd with something as simple as:

hg log `for x in $(seq 1 50) ; do echo "-r $x "; done`

Adding '1 + 2 + ... + 20' to the revsetbenchmark.txt didn't seem to repro the
problem, so I wasn't able to add a revset benchmark for this issue.

--HG--
branch : stable