[merge] add Repository.sprout

[Martin Pool]

In the thread about Repository.tarball I spoke about cleaning up
BzrDir.sprout and copy_content_into.

This is one change towards that: it adds a new Repository.sprout,
which I think is cleaner in intention and purpose than
copy_content_into.  It doesn't copy the extra settings of the
repository (like working-trees and shared), which is normally what
you'll want.

-- 
Martin