what if tracker url changes?

[Martin Pool]

I think the way forward is to do any or eventually all of the following:

* Document that the 'fixes' information is actually a more abstract
URI for a bug, not necessarily a URL through which you get an html
representation of it.
* Add an extension point by which we can get one or more urls (or more
abstract actions) given a bug URI: they might include http/html, api,
short human name, etc.  Use this when showing bugs.
* Define the emacs tracker builtin to bzr
* Make it easier to define new tracker shortcuts in the versioned tree
* Add a way to check an entered bug is valid/correct when entered (eg
by an api ping, showing the title)

-- 
Martin