rebase: delete seemingly unnecessary needupdate() This seemed to be about checking that the user hasn't updated away when we asked them to resolve merge conflicts. These days we call `cmdutil.checkunfinished()` and refuse to update, so the user shouldn't be able to get into this state. `test-rebase-interruptions.t` actually has some tests where it disables the rebase extension in order to be allowed to do some of these updates. That still passes, but I wouldn't personally haved cared if that failed. Differential Revision: https://phab.mercurial-scm.org/D7825

workingctx: move setparents() logic from localrepo to mirror overlayworkingctx It would be nice to later be able to call `wctx.setparents()` whether `wctx` is a `workingctx` or an `overlayworkingctx`. Differential Revision: https://phab.mercurial-scm.org/D7823

overlayworkginctx: implement a setparents() to mirror dirstate.setparents() This lets us make the in-memory and on-disk code a bit more similar. I'll soon also implement setparents() on the regular workingctx. Differential Revision: https://phab.mercurial-scm.org/D7822

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