Executing a parameter widget command calls a diff() or log()
of VcsBaseClient. The editor already existed, but
parameter widget is always created (call to setConfigurationWidget()
will be ignored in this case) - and parameter widget gets leaked.

Change-Id: I25591d5a54b80fed25ff11ce26108882c421e9fc
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>