unitig: the number of unitig vertices is deterministic

[boisvert@bigmem biosal]$ sha1sum run-1 run-2
06622cb9e6d8479b2bdd81c62d793e979d62ce2c  run-1
06622cb9e6d8479b2bdd81c62d793e979d62ce2c  run-2

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>