sg-report-flight: When justifying, disregard out-of-flight build jobs

sg-report-flight: When justifying, disregard out-of-flight build jobs

When we are looking for a previous job which justifies a failure, we
need to check that the putative justifying job used the same revision
as the baseline we are judging against.  So we investigate the build
jobs referred to by that putative justifying job.

However, there are cases where this is not appropriate.  Currently,
the most obvious example is freebsdbuildjob.

The freebsd arrangements involve referencing an anointed
freebsdbuildjob, which was in turn built from a previous
freebsdbuildjob, eventually terminating in some kind of ad-hoc job.
We would not want to investigate all of those even if we were trying
to analyse a flight on a freebsd branch.

The real thing we want to check that the build jobs *in the same
flight as the justifying job* used the right revisions.  Build jobs
from other flights were either (i) build jobs for components not being
targed for testing by this branch, but which were necessary for the
justifying job and for which we decided to reuse another build job
(in which case we don't really care what versions they used, even
if underlying it all there might be a different version of a tree
we are actually interested in (ii) the kind of continuous update
thing seen with freebsdbuildjob.

There is no corresponding issue with cs-bisection-step, because it
only looks for the *buildjob runvars of the test it is actually
interested in, and does not recurse.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>