Always allow force pushes to auxiliary Git repository
This addresses the bare minimum for #129 (closed)
This can be safely backported to Heptapod 0.7, because we
now have a way to be sure that this exception won't linger once
we support regular Git repos in the future. This was one of the
reasons to introduce the vcs_type
field.
I've been tempted to remove the check_push! completely for hg-git repos, knowing that we don't have any incoming Git calls from the outside. But this would have been more dangerous for the future.
(more details in the commit message)