[FIXED JENKINS-16654] Add option to disable changelog calculation

In some very very large repositories, the changelog calculation can take quite a bit of time. For users that do not care about seeing the changelog in Jenkins, this allows disabling the calculation, thus speeding up builds.

Our Jenkins infrastructure often needs to build older branches followed by newer branches and so on. On top of that, our Mercurial repository is extremely large. Calculating the changelog on this large repository can sometimes take 10-20 minutes. Since we don't really care about displaying the changelog in Jenkins (it becomes meaningless given the large number of commits), it would be great to disable this and decrease build times.

This is a second try of #52, based on Jesse's comments.

[JENKINS-16654](https://issues.jenkins-ci.org/browse/JENKINS-16654)