When a user invokes `git archive` with LFS files, `git lfs
filter-process` is invoked to smudge the LFS files. However, currently
when we instantiate the manifest object as part of that, an attempt is
made to connect to the remote using SSH, which we don't want to do
unless necessary. For example, if the user already has all the files
locally, the network connection is needless and serves only to waste
resources.
In the previous commit, we made our manifest an abstract interface with
a single implementing type: a concrete manifest. Now, introduce a lazy
manifest, which can upgrade to a concrete manifest but doesn't
instantiate one until that happens. This allows us to instantiate a
manifest without making the SSH connection, and we can delay the SSH
connection until it's really needed, if at all.
Add a test for this case as well.
a0065c0a48