test-scripts: Optimize check-copyright-comments.sh

[joshlf]

The test-scripts/check-copyright-comments.sh script is run as a git hook, and currently, it is slow enough that it meaningfully impacts the speed of running git commit or git rebase. We should either optimize it or provide an alternative script written in a faster language (e.g., Python) that we can use when support is available on the local platform.

[joshlf]

#190 Brings the execution duration from ~10.6s to ~4.1s. It's unclear how much more opportunity for optimization there is, since the majority of that time is taken in a single necessary git log call.