CF3 outputs almost nothing when run on 2505 human genomes

**Command line:**

```
$ /bin/cuttlefish build -m 128 -s references.fa -k 63 -t 128  -o unitigs -w workdir --ref -c 1
Constructing the compacted reference de Bruijn graph for k = 63.
Edge frequency cutoff: 1.
```

Waited for three days, then aborted. The input file is 6.6TB, 2505 human genomes. The same happens with the `--path-cover` flag.