Right now, creating a checkout object as used by the checkout and pull
commands creates a manifest.  This manifest in turn creates an
SSHTransfer object to potentially connect via the new pure SSH protocol.
However, connecting to the Internet during `git lfs checkout` is
unexpected and may cause this operation to fail when one is not online.

In order to set the manifest's configuration parameters correctly, we
need to know whether it's possible to make a pure SSH protocol
connection, so it's non-trivial to avoid this connection by delaying the
initialization of SSHTransfer object.  Instead, let's lazily create our
manifest in the checkout code to avoid creating the connection.  If
we're performing a `git lfs pull`, then it will be used and we'll
initialize it very quickly, making the connection, and if we're using
`git lfs checkout`, it won't be initialized at all, and we'll remain
offline.