context: default to using branch from dirstate only in workingctx

Authored by martinvonz.

Description

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