Change Cache_tag_rowid to a partial index because we don't need to query rows efficiently where tag IS NULL (grantjenks#307)

* Change `Cache_tag_rowid` to a partial index because we don't need to query rows efficiently `where tag IS NULL`

* Fix formatting

Author: ddorian

diskcache/core.py

@@ -2028,7 +2028,10 @@ def create_tag_index(self):
 
         """
         sql = self._sql
-        sql('CREATE INDEX IF NOT EXISTS Cache_tag_rowid ON Cache(tag, rowid)')
+        sql(
+            'CREATE INDEX IF NOT EXISTS Cache_tag_rowid ON Cache(tag, rowid) '
+            'WHERE tag IS NOT NULL'
+        )
         self.reset('tag_index', 1)
 
     def drop_tag_index(self):