rich-root vs subtree formats for 1.6

[Martin Pool]

At the moment we are introducing just these two new stable stacking
pack format names in 1.6:

format_registry.register_metadir('1.6',
    'bzrlib.repofmt.pack_repo.RepositoryFormatKnitPack5',
    help='A branch and pack based repository that supports stacking. ',
    branch_format='bzrlib.branch.BzrBranchFormat7',
    tree_format='bzrlib.workingtree.WorkingTreeFormat4',
    )
format_registry.register_metadir('1.6-rich-root',
    'bzrlib.repofmt.pack_repo.RepositoryFormatKnitPack5RichRoot',
    help='A branch and pack based repository that supports stacking '
         'and rich root data (needed for bzr-svn). ',
    branch_format='bzrlib.branch.BzrBranchFormat7',
    tree_format='bzrlib.workingtree.WorkingTreeFormat4',
    )

There is no 1.6-subtree, and neither of these formats have
supports_tree_reference turned on.  I think this is what we want, as
subtree is still experimental and there are experimental formats (like
development1-subtree) that do have it.  Is that correct?

-- 
Martin <http://launchpad.net/~mbp/>