Proposed fix for JENKINS-24401

The Depot class now has a method for obtaining the server's timezone GMT
offset. This offset is then applied to the changelist's submit time. Thanks to
that, the submit time of a changelist in the build's changelog appears in the
Jenkins server's timezone.
This change corrupts the backward compatibility. E.g. I use the constructure in several Groovy scripts
It makes sense to add the support of the default behavior when <code>depot==null</code>