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.
Done. The one-parameter constructor now initializes the depot field to null. The build() method then falls back to the default (non-timezoned) parsing method.