[FIXED JENKINS-24176] Avoid loading builds from disk just to display test result trend

Fixing [JENKINS-24176](https://issues.jenkins-ci.org/browse/JENKINS-24176) by:

* Making a fix similar to that of JENKINS-23945: stop looking backwards in trend graph when we seem to come to an unloaded build.
* Reverting fix of JENKINS-9839, since it is too expensive (as of Jenkins 1.485) to be worthwhile.