list: add option to show non-checkedout projects too

Currently, list only shows projects that exist in the checkout, and
doesn't offer any way to list all projects in the manifest (based on
the current settings, or on the options passed to list).  This seems
to be the opposite of what (at least some) users expect, so let's
add an option to show all of them regardless of checkout state.

Change-Id: I94bbdc5bd0ff2a411704fa215e7fc2b60fa3360e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/301263


Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>