Use commit hook scripts

[Uncommon]

Have options for running the various commit scripts:

• [ ] pre-commit: intended to run "first" before the message editor is opened. This is difficult in a modeless context, especially if the script may be time-consuming. Maybe have a button to run manually, and an indicator to show if the script ran successfully since the last local change.
• [ ] prepare-commit-msg: produces the initial commit message. Probably this should run when the message editor gains focus and is empty (or matches the template).
• [ ] commit-msg: validates the commit message. Running automatically when the user pauses typing is probably good.
• [ ] post-commit: after the commit is complete.