travis: git clone at great depth
travis: git clone at great depth

ensures that git commits referenced by backwards compatibility tests are
always available in CI.

containernetworking/plugins,appc/cni,mcastelino/cni,rosenhouse/cni,dcbw/cni,squeed/plugins,containernetworking/cni,kad/cni,appc/cni,rosenhouse/cni,kad/cni,squeed/plugins,containernetworking/cni,tangle329/cni,mcastelino/cni,tangle329/cni,containernetworking/plugins,dcbw/cni