Trusted commit authors hook timeout when there are a large number of tags

In SRBITB-176 we excluded tags when we read the commits in the trusted commit authors hook as they are already in the repository.

This was to handle merges from a tag, which is quite a niche use case.

With many thousands (11,000) of tags in the repository this causes the hook to timeout or the push takes many minutes to finish even when just pushing a single commit. It only takes a few seconds to finish when there are only a few thousand tags.

You only really want to know if the commit exists anywhere else in the repository. Therefore we can exclude the commit if it already exists in the index. The index should in almost all cases be up to date.