fix. bug in revert_rate.  Historics revs should be ordered by descending rev_id.
add. test for RevertRate.

Change-Id: Ia93567569c604564c30a3e76371c03570b2305b7
add. test for RevertRate.

Change-Id: I0f88fffa3069fe642245d7af658ac7d53e5e778f