[MERGE] Add format registry for send formats.

[Ian Clatworthy]

Jelmer Vernooij wrote:

> Like my bzr serve --protocol patch, it puts a registry inside a cmd_
> implementation (cmd_send.format_registry) but I think this is the
> cleanest solution.

Again, I'd prefer not having this sort of registry in the UI layer.
Putting it in the merge_directive module is a better choice IMO though
I'm open to other suggestions, e.g. creating a send.py module, moving
most of the logic out of cmd_send into there and also putting the
registry there.

bb:resubmit

Ian C.