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