changectx: ancestor should only prefer merge.preferancestor if it is a revision

The value '*' currently designates that bid merge should be used. The best
way to test bid merge is to set preferancestor=* in the configuration file ...
but then it would abort with unknown revision '*' when other code paths ended
up in changectx.ancestor .

Instead, just skip and ignore the value '*' when looking for a preferred
ancestor.

mercurial/context.py

@@ -553,6 +553,8 b' class changectx(basectx):'
             anc = cahs[0]
         else:
             for r in self._repo.ui.configlist('merge', 'preferancestor'):
+                if r == '*':
+                    continue
                 ctx = changectx(self._repo, r)
                 anc = ctx.node()
                 if anc in cahs:

tests/test-merge-criss-cross.t

@@ -341,4 +341,15 b' http://stackoverflow.com/questions/93500'
   b
   c
 
+Verify that the old context ancestor works with / despite preferancestor:
+
+  $ hg log -r 'ancestor(head())' --config merge.preferancestor=1 -T '{rev}\n'
+  1
+  $ hg log -r 'ancestor(head())' --config merge.preferancestor=2 -T '{rev}\n'
+  2
+  $ hg log -r 'ancestor(head())' --config merge.preferancestor=3 -T '{rev}\n'
+  1
+  $ hg log -r 'ancestor(head())' --config merge.preferancestor='*' -T '{rev}\n'
+  1
+
   $ cd ..