repo: move unfiltered-repo optimization to workingctx localrepo.__getitem__ special-cased lookup of the working copy parent to avoid looking up obsmarkers. I think the reason for that code (which I once wrote myself) was to make `hg commit` not load obsmarkers, which it would otherwise do via ctx.p1() in localrepo.commitctx(). That had the somewhat unfortunate consequence of making lookup of an unrelated binary nodeid load the dirstate. Now that changectx's constructor is dumb, we can let workingctx._parents() have the opmtimization instead. This affects two tests, because they no longer end up loading the dirstate and their "warning: ignoring unknown working parent ..." messages therefore go way. Differential Revision: https://phab.mercurial-scm.org/D4828