[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.
Jesse,

Doe this mean that if the previous build was deleted, the graph won't be displayed? Can't this also be handled by limiting the builds displayed it the graph to the 10 or 20 most recent builds?