hooks for commit message editor

[Martin Pool]

On Elliot Murphy writes in bug 97660:

> Directly related to Andrew's comment about per-project policy, I would
> very much like the ability to customize what is displayed in this
> message for some of my projects, reminding people about commit message
> policies or formats.

We could have a hook that can return both default text, and also comments. We could also have a hook that updates or validates the message that has been entered to make sure it fits local policy (like word wrapping it or checking for a reviewer.)