fixed formula for conflict score

To make use of all matches, we define:

  P(conflict) = 1 - L_0 / sum_i L_i

where L_0 is the likelihood of the best match and L_i is the likelihoods
of further matches.  Computation is done in log space to keep the
precision:

  log P(conflict) = Oplus_{i=1}^n log L_i - Oplus_{i=0}^n log L_i

where Oplus is the addition in log space.