merge: make low-level update() private (API) We have very few callers left that call the low-level `merge.update()` function. I think it's time to make it private. I'll remove the remaining callers in coming patches, except for one call from the `rebase` module. I hope to eventually fix that too, but it's more complex because it requires teaching `merge.graft()` to work with a dirty working copy. Differential Revision: https://phab.mercurial-scm.org/D9065

merge: use merge.clean_update() when applicable We have had this higher-level function (higher than `merge.update()`, that is) for a while. Let's simply some callers by using it. I don't know why I didn't do this when I introduced the function. After this patch, there are no remaining callers that call `hg.updaterepo()` with `overwrite=True`. We'll clean that up soon. Differential Revision: https://phab.mercurial-scm.org/D9063

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