commands: inline definition of localrepo.parents() and drop the method (API) localrepo.parents() has relatively few users, and most of those were actually implicitly looking at the wctx, which is now made explicit via repo[None].

localrepo: document nodebookmarks

localrepo: remove clone method by hoisting into hg.py hg.py was the only remaining caller of localrepo.clone(), so it's time to move some more behavior out of localrepo.