[FIXED JENKINS-23309] changes are now displayed for upstream jobs residing within the same folder

The only problem with the former implementation was that, UpstreamCause.getUpstreamProject() providing a "full name", Hudson.getItem() was returning null if upstream job happened to be located in the same folder as the downstream one.

The proposed fix just consists in using Hudson.getItemFullName() instead. No risk for flat structures, i.e. not using folders: "full name" would equal to "short name".

For more information, please have a look at: https://wiki.jenkins-ci.org/display/JENKINS/Hierarchical+projects+support