repo: avoid cyclic dependency with is_remote method

repo: avoid cyclic dependency with is_remote method

The current VFS operations (like resolve()) require a repo object, but
we're about to add a VFS resolve() method to the repos.  In and of
itself, that isn't necessarily a problem, but as an optimization, we
want the VFS resolve() to be able to detect when the repo it's been
given is a RemoteRepo and redirect the call to remote_repo.resolve().

Doing so pushes the one single resolve() call to the remote instead of
executing the resolve() locally with a lot of individual calls to the
remote_repo's other methods.

Adding is_remote() makes that possible without having to 'import repo'
in the VFS (repo already imports vfs).

Perhaps we'll rework it later, but this will do for now.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>