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.
Wrong coding-style.
It also makes sense to add Javadoc and/or <code>@Nonnull</code> or <code>@CheckForNulll</code> annotations to prevent the improper usage.