Add support for using tarball snapshots for git-clone.

This doesn't allow for resuming at this point but if it's not too hard
to add and there's interest from the sysadmins I can look into that too.

Either way it should allow for a load reduction on git.kde.org (or
rather, anongit.kde.org), but this only works for modules accessed
through the projects.kde.org XML database (i.e. repository kde-projects
in your module-set)