Avoid MAPQ=60 for single-end multimappers

by ensuring that we compute at least two alignments even if the first that
is found has edit distance 0.

See #293

src/aln.cpp

@@ -100,7 +100,7 @@ static inline void align_SE(
     int min_mapq_diff = best_edit_distance;
     for (auto &nam : nams) {
         float score_dropoff = (float) nam.n_hits / n_max.n_hits;
-        if (tries >= max_tries || best_edit_distance == 0 || score_dropoff < dropoff_threshold) {
+        if (tries >= max_tries || (max_tries > 1 && best_edit_distance == 0) || score_dropoff < dropoff_threshold) {
             break;
         }
         bool consistent_nam = reverse_nam_if_needed(nam, read, references, k);