[MERGE] Add a new hook Commands['extend_command'] for plugins that want to alter commands without overriding the entire command.

[Andrew Bennetts]

Andrew Bennetts has voted tweak.
Status is now: Conditionally approved
Comment:
I'm not convinced that this is the best way to enable plugins to extend 
things, but I think it's worth having despite that.  It's good to allow 
cheap experimentation, even if it's a bit hackish.  The uses people put 
this to will probably suggest better hooks for us to add.

I think plugins using this will tend to be a little more fragile than 
most, though, so perhaps we should warn a little more about that in the 
docs.

Other than that, one typo you should fix:

> +        # Introduced in 0.13:

ITYM 1.13 :)

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C1234384804.12070.622.camel%40lifeless-64%3E
Project: Bazaar