merge: fix error message

Obvious copy-and-paste error

mercurial/merge.py

@@ -311,7 +311,7 b' class mergestate(object):'
     @util.propertycache
     def otherctx(self):
         if self._other is None:
-            raise RuntimeError("localctx accessed but self._local isn't set")
+            raise RuntimeError("otherctx accessed but self._other isn't set")
         return self._repo[self._other]
 
     def active(self):