copyright-notice problem with shallow clones

During GitLab-CI pipelines, the copyright-notice hook consistently started reporting a file as not up-to-date, despite the hook passing every time it was run manually.

In a shallow clone, any file which was last changed before a grafted commit ends up taking the date of the grafted commit as it's last update time.

To fix, files that are added in a grafted commit should be ignored by the hook.

To find grafted commits: read each line of $(git rev-parse --git-dir)/shallow