configitems: register the 'merge.preferancestor' config

mercurial/configitems.py

@@ -302,6 +302,9 b" coreconfigitem('http_proxy', 'user',"
 coreconfigitem('merge', 'followcopies',
     default=True,
 )
+coreconfigitem('merge', 'preferancestor',
+        default=lambda: ['*'],
+)
 coreconfigitem('pager', 'ignore',
     default=list,
 )

mercurial/context.py

@@ -659,7 +659,7 b' class changectx(basectx):'
             anc = cahs[0]
         else:
             # experimental config: merge.preferancestor
-            for r in self._repo.ui.configlist('merge', 'preferancestor', ['*']):
+            for r in self._repo.ui.configlist('merge', 'preferancestor'):
                 try:
                     ctx = changectx(self._repo, r)
                 except error.RepoLookupError:

mercurial/merge.py

@@ -1566,7 +1566,7 b' def update(repo, node, branchmerge, forc'
 
         p2 = repo[node]
         if pas[0] is None:
-            if repo.ui.configlist('merge', 'preferancestor', ['*']) == ['*']:
+            if repo.ui.configlist('merge', 'preferancestor') == ['*']:
                 cahs = repo.changelog.commonancestorsheads(p1.node(), p2.node())
                 pas = [repo[anc] for anc in (sorted(cahs) or [nullid])]
             else: