mportsync: Ignore top-level directory in snapshots
mportsync: Ignore top-level directory in snapshots

The name of the top-level directory in a snapshot tarball had to be the
same as the root name of the tarball, which means the file name without
extension.

However, tarballs generated by GitHub do not follow this rule, as it
uses a top-level directory name like "repository-branch/". Therefore,
this will ignore the top-level directory in the tarball and always use
the root name as directory name for extraction. This is fully compatible
with existing snapshot tarballs as the tarball root name and top-level
directory name matched anyway.

This change allows to to sync the ports tree directly from GitHub as
a tarball updated after every commit by using this line in sources.conf:

https://github.com/macports/macports-ports/archive/master/ports.tar.gz [default]

macports/macports-base,neverpanic/macports-base,danchr/macports-base,neverpanic/macports-base,danchr/macports-base,macports/macports-base