Windows: cm_MergeStatus use new DV to purge buffers

Windows: cm_MergeStatus use new DV to purge buffers

When deciding whether or not to purge buffers on a DV change
it is the new DV that matters not the old DV.  If the new DV
is 0, there should be no purging because there are no buffers
to purge.

Change-Id: Ie505348dbbf8173e477645e5022410ddd3028240
Reviewed-on: http://gerrit.openafs.org/6176
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>