context: default to using branch from dirstate only in workingctx
Same reasoning as previous commits: only the workingctx should know
about the dirstate.
committablectx now seems free of dirstate references.
Differential Revision: https://phab.mercurial-scm.org/D6374
public attribute not declared in interfaces: badpeer.badattribute
public attribute not declared in interfaces: badpeer.badmethod