Use VcsBaseClient::diff instead of svn's one

The first step in cleaning a mess in VCS

Change-Id: I06ee1d9952cc91dc8f599f906d221850ffb0835b
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
Use VcsBaseClient::diff instead of cvs's one

The second step in cleaning a mess in VCS

Change-Id: I3eb360825480c74242110b0da9f90b39fc4c767f
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>