merge: get the default update destination from the function

There is no value in using the revset instead of the extracted function.

mercurial/merge.py

@@ -21,6 +21,7 b' from .node import ('
 )
 from . import (
     copies,
+    destutil,
     filemerge,
     obsolete,
     subrepo,
@@ -994,9 +995,7 b' def update(repo, node, branchmerge, forc'
             pas = [repo[ancestor]]
 
         if node is None:
-            nodes = list(repo.set('_updatedefaultdest()'))
-            if nodes:
-                node = nodes[0].node()
+            node = repo[destutil.destupdate(repo)].node()
 
         overwrite = force and not branchmerge