scm: prevent diff shows trace if UTF-8 is invalid in Ruby 1.9.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@4912 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/helpers/repositories_helper.rb

@@ -132,6 +132,10 @@ module RepositoriesHelper
         # do nothing here and try the next encoding
       end
     end
+    if str.respond_to?(:force_encoding)
+      str = str.encode("ASCII-8BIT", :invalid => :replace,
+              :undef => :replace, :replace => '?').encode("UTF-8")
+    end
     str
   end