mercurial/hg.py

--- a/mercurial/hg.py	Mon Feb 13 12:58:37 2017 -0800
+++ b/mercurial/hg.py	Mon Feb 13 16:03:05 2017 -0800
@@ -747,7 +747,10 @@
     This returns whether conflict is detected at updating or not.
     """
     if updatecheck is None:
-        updatecheck = 'linear'
+        updatecheck = ui.config('experimental', 'updatecheck')
+        if updatecheck not in ('abort', 'none', 'linear'):
+            # If not configured, or invalid value configured
+            updatecheck = 'linear'
     with repo.wlock():
         movemarkfrom = None
         warndest = False