mergestate: raise exception if otherctx is accessed but _other isn't set

We don't want to inadvertently return the workingctx (self._repo[None]).

mercurial/merge.py

@@ -288,6 +288,8 b' class mergestate(object):'
 
     @util.propertycache
     def otherctx(self):
+        if self._other is None:
+            raise RuntimeError("localctx accessed but self._local isn't set")
         return self._repo[self._other]
 
     def active(self):