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