merge: make merge.preferancestor type and default consistent

(and mark it)

mercurial/context.py

@@ -576,7 +576,8 b' class changectx(basectx):'
         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:

mercurial/merge.py

@@ -1011,7 +1011,7 b' def update(repo, node, branchmerge, forc'
 
         p2 = repo[node]
         if pas[0] is None:
-            if repo.ui.config('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: