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
Ancestors of 5
4 2 0
Ancestors of 6 and 5
4 3 2 1 0
Ancestors of 5 and 4
4 2 0
Ancestors of 7, stop at 6
6
Ancestors of 7, including revs
7 6 5 4 3 2 1 0
Ancestors of 7, 5 and 3, including revs
7 6 5 4 3 2 1 0
Descendants of 5
7 8
Descendants of 5 and 3
6 7 8
Descendants of 5 and 4
5 7 8