prepman: do not compress files <=128 bytes

The vast majority of these small files do not compress better than
their inputs, and they're just .so redirection.  Omit compression
on them to save disk and cpu and speed things up.

URL: http://bugs.gentoo.org/169260
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
resolver: output: check for None mainRepo()

mainRepo() can return none when we are on a machine that is only using
PORTAGE_BINHOST and not a normal PORTAGE. Make sure we don't crash here.