[MERGE][RFC] Enhanced hooks

[Ian Clatworthy]

Robert Collins wrote:

> Basically I think what you have done is turned a self-registered system
> into a manunally registered system. And this is why it is cumbersome to
> work with.
> 
> I suggest you approach the problem differently and ask yourself 'how can
> I get a hook written in shell to auto-register a python callable for
> e.g. branch'.

I haven't removed any of the (implicitly global) self-registration -
I've added selective registration so the active hooks can differ (for
example) per branch.

Let's me know on IRC what API you want given the current branch is now
needed to find the list of hooks.

Ian C.