Add repo abstraction and use it in join

Add repo abstraction and use it in join

Operations that can be --remote currently rely on code like this:

  if opt.remote:
      cli = client.Client(opt.remote)
      cat = cli.cat
  else:
      cp = git.CatPipe()
      cat = cp.join

Instead, add LocalRepo and RemoteRepo classes with matching methods so
that we can say:

  repo = RemoteRepo(opt.remote) if opt.remote else LocalRepo()

and then use repo methods to handle the work.

Rework "bup join" accordingly.

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