Diagnosis for possible race condition in LogRotator when using external-monitor-job.
java.io.IOException: …/builds/2013-07-11_12-21-02 is in use
    at hudson.model.Run.delete(Run.java:1363)
    at hudson.tasks.LogRotator.perform(LogRotator.java:133)
    at hudson.model.Job.logRotate(Job.java:406)
    at hudson.model.Run.execute(Run.java:1638)
Merge branch 'master' of github.com:jenkinsci/jenkins