dirstate: move parents source of truth to dirstatemap
As part of moving dirstate storage to its own class, let's move the source of
truth for the dirstate parents to dirstatemap. This requires that dirstate._pl
no longer be a cache, and that all sets go through dirstatemap.setparents.
Differential Revision: https://phab.mercurial-scm.org/D759
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}