dirstate: add some traces on listdir calls I'm starting to spend some time trying to make `hg status` do less work, and knowing what's happening in here and what's expensive is an important start. Differential Revision: https://phab.mercurial-scm.org/D7109

cleanup: hgdemandimport.tracing accepts strings, not bytes This does mean that the resulting traces will have some b'' goo in them on Python 3, but I think that's a worthwile price to pay since it doesn't break any tests. Differential Revision: https://phab.mercurial-scm.org/D7108