verify-subtree.sh: relax check and ignore old content
verify-subtree.sh: relax check and ignore old content

If for whatever reasons a repo already had a `release-tools` directory
before doing a clean import of it with `git subtree`, the check used
to fail because it found those old commits.

This can be fixed by telling `git log` to stop when the directory
disappears from the repo. There has to be a commit with removes the
old content, because otherwise `git subtree add` doesn't work.

Fixes: https://github.com/kubernetes-csi/external-resizer/issues/21

kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-lib-utils,kubernetes-csi/external-attacher-csi,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs