A per-repo, untracked `.gitignore-local`

You have files that you want to ignore, but that have no business being in the project's .gitignore: a a local test script, some draft assets. Adding them to .gitignore would mean committing your personal clutter for everyone. Leaving them alone means git status keeps nagging about untracked files. Git has a built-in answer for this, .git/info/exclude, but it's tucked…