Allow aborting a commit message without committing "Co-authors" tag

[ryanjwessel]

Current Behavior

Currently, we use prepare-commit-msg to include "Co-authors: {...coauthors}` in the commit body. However, if the author exits the commit editor, because there is a non-empty string included in the body, it counts as an actual commit message in your history.

Desired Behavior

If we included a commit-msg hook that checked if the body contains only empty lines and the Co-authors: tag, then we know the commit is incomplete.

Credit to @andersonvom for discovering this issue!