specific pins below 1000 cause downgrades

specific pins below 1000 cause downgrades

We start your quest by using the version of a package applying to a
specific pin, but that version could very well be below the current
version, which causes APT to suggest a downgrade even if it is
advertised that it never does this below 1000.

Its of course questionable what use a specific pin on a package has
which has a newer version already installed, but reacting with the
suggestion of a downgrade is really not appropriated (even if its kinda
likely that this is actually the intend the user has – it could just as
well be an outdated pin) and as pinning is complicated enough we should
atleast do what is described in the manpage.

So we look out for the specific pin and if we haven't seen it at the
moment we see the installed version, we ignore the specific pin.

Closes: 543966