[MERGE] Use registry for revision specifiers

[John Arbash Meinel]

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Jelmer Vernooij wrote:
> The attached patch changes the revision specifiers to use a registry
> rather than a list of items. This allows lazy registration of revision
> specifiers, such as "svn:". 
> 
> Revision specifiers and control formats are the only two things that
> can't be lazily registered yet and that trigger loading of more than
> just bzrlib.plugins.svn.__init__ when running (non-svn) bzr operations
> and bzr-svn is installed.
> 
> Cheers,
> 
> Jelmer
> 

If we wanted to be nice and allow backwards compatibility, we could
still allow for SPEC_TYPES, and just use the deprecated_list I wrote a
long time ago:

 bzrlib.symbol_versioning.deprecated_list

I would like to do the backwards compatible thing, especially since it
means that older versions of things like bzr-svn won't just die because
a variable they expect isn't there

BB:tweak

Other than that, it looks good.

John
=:->
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