Fix bug #845957 "exception when viewing a revision without log"

reported by Hernan Martinez Foffani.

* lib/viewcvs.py
  (view_markup):
    set default value for "tag" template variable


git-svn-id: http://viewvc.tigris.org/svn/viewvc/trunk@778 8cb11bc2-c004-0410-86c3-e597b4017df7

lib/viewcvs.py

@@ -963,6 +963,7 @@ def view_markup(request):
                                   params={'content-type': 'text/plain'}),
     'mime_type' : request.mime_type,
     'log' : None,
+    'tag' : None,
     })
 
   if cfg.options.show_log_in_markup: