mercurial/context.py

--- a/mercurial/context.py	Thu Jun 25 17:53:16 2015 -0500
+++ b/mercurial/context.py	Thu Jun 25 17:54:55 2015 -0500
@@ -576,7 +576,8 @@
         elif len(cahs) == 1:
             anc = cahs[0]
         else:
-            for r in self._repo.ui.configlist('merge', 'preferancestor'):
+            # experimental config: merge.preferancestor
+            for r in self._repo.ui.configlist('merge', 'preferancestor', ['*']):
                 try:
                     ctx = changectx(self._repo, r)
                 except error.RepoLookupError: