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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}