Don't clone the whole history if we need just the tip of a single branch

* packages/gen-tarball.sh: Clone only the tip of the required branch for
non-GnuPG repos.

This speeds up cloning of non-GnuPG repos by several magnitudes and
saves loads of bandwidth. For GnuPG repos we keep cloning the whole
history because we need an unknown amount of history for calculating
the beta version number.