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