cluster-autoscaler/hack/update-vendor.sh: fix usage of git commit
cluster-autoscaler/hack/update-vendor.sh: fix usage of git commit

Running this script as-is fails as `git ci` is not a recognised
command. Fixed by referencing `git commit` explicitly; I guess it
works as it may be aliased.

kubernetes/autoscaler,mmerrill3/autoscaler,kubernetes/autoscaler,kubernetes/autoscaler,mmerrill3/autoscaler,bskiba/autoscaler,mmerrill3/autoscaler,bskiba/autoscaler,kubernetes/autoscaler,bskiba/autoscaler,mmerrill3/autoscaler