ci: Use shallow clones
ci: Use shallow clones

This speeds up the preparation step.

Note that the DCO job needs to use a slighly less shallow clone
than the other jobs, because it's entirely possible that someone
would submit a merge request with more than 100 patches in it.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

libvirt/libvirt-glib,libvirt/libvirt-glib,libvirt/libvirt-glib