improve artifact_feed.author_link index by including field it is sorted on
diff --git a/Allura/allura/model/artifact.py b/Allura/allura/model/artifact.py
index 487530f..4ec0532 100644
--- a/Allura/allura/model/artifact.py
+++ b/Allura/allura/model/artifact.py
@@ -898,7 +898,8 @@
              ('app_config_id', pymongo.ASCENDING),
              ('pubdate', pymongo.DESCENDING)),
             # used in ext/user_profile/user_main.py for user feeds
-            'author_link',
+            (('author_link', pymongo.ASCENDING),
+             ('pubdate', pymongo.DESCENDING)),
             # used in project feed
             (('project_id', pymongo.ASCENDING),
              ('pubdate', pymongo.DESCENDING)),