sg-report-flight: Actually look at retest flights (part 1)

sg-report-flight: Actually look at retest flights (part 1)

The existing approach does not find retest flights.  This is because
it starts by looking for flights whose runvars say they built the
version in question, and then looks to see if they contain the
relevant job.

Retest flights don't contain build jobs; they reuse the builds from
the primary flight.

Rather than making a fully general recursion scheme (which would
involve adding another index so we could quickly find all flights
which refer to this one), we add a one-level recursion.

This recursion is restricted to the flights named on the command line.
This means it takes nearly no time (as opposed to searching the whole
db for things that might be relevant - see above re need for an
index).

We filter the command line flights, looking ones which refer to the
only the primarily found flights as build jobs.

Signed-off-by: Ian Jackson <iwj@xenproject.org>