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