lpsetup: get command

(Copied from the kanban card.)

Gets a full tree (LP, download-cache, source).
Supports lightweight checkouts (default) or "normal"/co-located branches.
Create a new subcommand in `lpsetup/subcommands`.
Idempotent (nice to have).
It should be subdivided in steps: i.e use `StepsBasedSubcommand`.
Subsumes *lp-setup update* (see below).

Optional arguments:
    repository (positional): Default is ~/launchpad.
    branch (positional): Default is "devel".
    -s --source: Default is `lp:launchpad` (if --use-http is false).
        or `http://bazaar.launchpad.net/~launchpad-pqm/launchpad/devel`
    --use-http (false): Use http in place of bzr+ssh, ignored if --source is
        provided.
    -t --with-trees: Use normal branches.
    --checkout (sandbox): The name of the lightweight checkout to be created,
        ignored if --with-trees is true.
    -u --user: The name of the system user used to get Launchpad.
        The current user is used if this script is not run as root
        and this argument is omitted.
    decision needed: --private-key, --public-key, --ssh-key-name?

E.g. without trees::

 bzr init-repo --no-trees launchpad &&
 cd launchpad &&
 bzr branch lp:launchpad devel &&
 bzr co --lightweight devel sandbox &&
 call lp-setup update on sandbox

E.g. with trees::

 bzr init-repo launchpad &&
 cd launchpad &&
 bzr branch lp:launchpad devel &&
 call lp-setup update on devel