Re: sqlsmith crash incremental sort

I've pushed fix with the DEFAULT_NUM_DISTINCT. The input comes from a
set operation (which is where we call generate_append_tlist), so it's
probably fairly unique, so maybe we should use input_tuples. But it's
not guaranteed, so DEFAULT_NUM_DISTINCT seems reasonably defensive.

One detail I've changed is that instead of matching the expression
directly to a Var, it now calls pull_varnos() to also detect Vars
somewhere deeper. Lookig at examine_variable() it calls find_base_rel
for such case too, but I haven't tried constructing a query triggering
the issue.

One improvement I can think of is handling lists with only some
expressions containing varno 0. We could still call estimate_num_groups
for expressions with varno != 0, and multiply that by the estimate for
the other part (be it DEFAULT_NUM_DISTINCT). This might produce a higher
estimate than just using DEFAULT_NUM_DISTINCT directly, resulting in a
lower incremenal sort cost. But it's not clear to me if this can even
happen - AFAICS either all Vars have varno 0 or none, so I haven't done
this.