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