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
Differential Revision: https://phab.mercurial-scm.org/D9076