don't fail if /var/lib/puppet-git-receiver.git already exists

This is a simple check; only prevents the package install/upgrade from failing
(from dpkg's perspective) by discarding any error during the linkage.

I'm reasonably sure that 99%+ of the time this is the correct approach; and
that the other 1% I really don't want to be pre-guessing :)
Merge branch 'master' into ppa-base

* master:
  don't fail if /var/lib/puppet-git-receiver.git already exists
puppet-git-receiver (1.5.0-1.2) UNRELEASED; urgency=low