overlayworkingctx: default branch to base context's branch...
overlayworkingctx: default branch to base context's branch
This matches what the dirstate does (it reuses working copy parent's
branch unless told otherwise). By moving the default out of
`rebase.commitmemorynode()`, it will let us clean that up better
later.
Differential Revision:
https://phab.mercurial-scm.org/D7821
