Prevent the config-lint script erroring out on any sample_config changes (#9562)

I noticed that I'd occasionally have `scripts-dev/lint.sh` fail when messing about with config options in my PR. The script calls `scripts-dev/config-lint.sh`, which attempts some validation on the sample config.

 It does this by using `sed` to edit the sample_config, and then seeing if the file changed using `git diff`.

The problem is: if you changed the sample_config as part of your commit, this script will error regardless.

This PR attempts to change the check so that existing, unstaged changes to the sample_config will not cause the script to report an invalid file.