BzrDir.cloning_metadir should delegate to the individual objects

BzrDir.cloning_metadir is quite nice, but it can be tricky when you have
things like lightweight checkouts or shared repositories to be sure that
the right format is created. It looks like it would be easier to get
this right if the cloning_format was delegated down to the individual
objects - perhaps like:

result._repository_format = source_repository.cloning_format(result)

which would allow selection based on:
 the bzrdir type
 the url (and thus local/remote issues)

while handling cleanly the case there the bzrdir type (and thus the
appropriate format) is when the bzrdir for the repository and branch, or
branch and tree differs.

 affects /products/bzr
 status confirmed
 importance medium

Rob
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