Add a pre-commit hook to expand relative ticket refs to absolute

[bswck]

The hook would possibly change commit messages to ensure absolute ticket refs.

E.g.

• #27 → jaraco/skeleton#27, if we commit in skeleton,
• #27 → jaraco/tempora#27, if we commit in tempora.

Addresses commit integrations mismatch.

[jaraco]

Yes, that sounds wise!

I'm a little worried about a pre-commit hook, because it requires the developer to have the hook installed (which in turn requires the developer to have the tool installed somewhere) and doesn't affect contributions through GitHub. What I'd like more is something like branch protection that would reject a push that mentions a bare issue number.