code: public @Nonnull RangeSet getRangeSet(String jobFullName) {
          RangeSet r = usages.get(jobFullName);
          if(r==null) r = new RangeSet();
          return r;
      }
docs: Gets the build range set for the given job name.
  
  <p>
  These builds of this job has used this file.
  @return may be empty but not null.