[FIX JENKINS-18048] Fix link of SCMTriggerCause from UpstreamCause

- UpstreamCause now tries to initialize upstream causes with the
  correct build in onLoad
- SCMTriggerCause stores build reference and uses its URL to build
  absolute link to changelog
Merge branch 'master' into JENKINS-18048

Conflicts:
	core/src/main/java/hudson/triggers/SCMTrigger.java
Merge pull request #1250 from daniel-beck/JENKINS-18048

[FIX JENKINS-18048] Fix link of SCMTriggerCause from UpstreamCause