changectx: skip all invalid merge.preferancestor values

A better fix for 17011b36aac7 that will ignore other kinds of "invalid"
revisions.

mercurial/context.py

@@ -558,9 +558,10 b' class changectx(basectx):'
             anc = cahs[0]
         else:
             for r in self._repo.ui.configlist('merge', 'preferancestor'):
-                if r == '*':
+                try:
+                    ctx = changectx(self._repo, r)
+                except error.RepoLookupError:
                     continue
-                ctx = changectx(self._repo, r)
                 anc = ctx.node()
                 if anc in cahs:
                     break

tests/test-merge-criss-cross.t

@@ -349,7 +349,7 b' Verify that the old context ancestor wor'
   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
+  $ hg log -r 'ancestor(head())' --config merge.preferancestor='1337 * - 2' -T '{rev}\n'
+  2
 
   $ cd ..