[merge] split out repository formats

[Martin Pool]

One thing I hit in doing this is that some registries (such as
BzrDirFormat) hold classes, while others (such as RepositoryFormat)
hold instances of the format object. 

Now personally I would rather in most cases dispense with the Format
object and class altogether and just hold the actual Repository class or
whatever.

Failing that maybe they should be made consistent.

-- 
Martin