scm: mercurial: fix unit adapter annotate test fails on Windows Mercurial 1.8.4+29-e597ef52a7c2.

git-svn-id: svn://rubyforge.org/var/svn/redmine/trunk@6042 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/redmine/scm/adapters/mercurial_adapter.rb

@@ -271,7 +271,8 @@ module Redmine
           end
           blame
         rescue HgCommandAborted
-          nil  # means not found or cannot be annotated
+          # means not found or cannot be annotated
+          Annotate.new
         end
 
         class Revision < Redmine::Scm::Adapters::Revision