context: let caller pass in branch to committablectx.__init__() committablectx.__init__() currently looks up the branch from the dirstate unless it's passed in the extras. memctx.__init__() has a branch argument, but since committablectx.__init__() doesn't accept it, it lets that constructor look up the branch from the dirstate before it overwrites it, which seems awkward. Differential Revision: https://phab.mercurial-scm.org/D6366