narrowspec: add timing block for validating narrowspec This was showing up in an operation I was doing today, and I'd like to be able to get trace spans for it instead of just profiler samples. Differential Revision: https://phab.mercurial-scm.org/D12185

  $ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
  Mercurial Distributed SCM (version *) (glob)