merge: first part of fix for issue1327

When there are no renames involved, we shortcut to the changeset
ancestor. This resolves most cases.

Note that Mercurial's rename philosophy elsewhere is that a file's
name is signficant and rename data is only consulted when a file of
the same name is absent.

mercurial/context.py

@@ -451,6 +451,10 b' class filectx(object):'
         find the common ancestor file context, if any, of self, and fc2
         """
 
+        actx = self.changectx().ancestor(fc2.changectx())
+        if self.path() in actx:
+            return actx[self.path()]
+
         acache = {}
 
         # prime the ancestor cache for the working directory