Seed tablesample strategy

This produces stable approximate counts among invocations unless the
table changed.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58791.

lib/gitlab/database/count/tablesample_count_strategy.rb

@@ -61,6 +61,7 @@ module Gitlab
             SELECT (COUNT(*)*#{inverse})::integer AS count
             FROM #{model.table_name}
             TABLESAMPLE SYSTEM (#{portion * 100})
+            REPEATABLE (0)
             #{where_clause(model)}
           SQL