rebase: teach in-memory rebase to not restart with on-disk rebase on conflict When in-memory rebase runs into conflicts, it redoes the whole rebase operation. This patch teaches it to instead discard just the current `overlayworkingctx` and redo that node on disk. I've tested this by enabling in-memory rebase by default and checking that there are no unexpected differences after this patch. The next step is to make it so that `hg rebase --continue` can use in-memory merge. Differential Revision: https://phab.mercurial-scm.org/D9076