Test the format version upgrade code

This test contains tarballs of repositories created with older
versions of StGIT. It also contains the script used to generate them,
but at some point we will lose the ability to easily run old versions
-- for example, if git changes incompatibly -- so tarballs will be the
only practical option for sufficiently old versions.

Signed-off-by: Karl Hasselström <kha@treskal.com>