update: check wc.dirty() before setting overwrite=True

merge.update() was missing a few dirtiness checks from workingcontext,
including subrepo cleanliness checks. Using wc.dirty() instead of
one-off checks for various forms of dirtiness will be significantly
safer.

mercurial/merge.py

@@ -525,7 +525,7 b' def update(repo, node, branchmerge, forc'
         elif not overwrite:
             if pa == p1 or pa == p2: # linear
                 pass # all good
-            elif wc.files() or wc.deleted():
+            elif wc.dirty(missing=True):
                 raise util.Abort(_("crosses branches (merge branches or use"
                                    " --clean to discard changes)"))
             elif onode is None: