merge.update: use `first` instead of direct indexing

This makes it compatible with all smartset classes.

mercurial/merge.py

@@ -1041,7 +1041,7 b' def update(repo, node, branchmerge, forc'
 
                     # get the max revision for the given successors set,
                     # i.e. the 'tip' of a set
-                    node = repo.revs("max(%ln)", successors)[0]
+                    node = repo.revs("max(%ln)", successors).first()
                     pas = [p1]
 
         overwrite = force and not branchmerge