checkout: permit checkout of a single file

fixes a coredump when using a command like:

$ ostree --repo=repo checkout -U --subpath=/usr/lib/passwd \
  fedora-atomic/rawhide/x86_64/docker-host usrlib-new

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

tests/test-repo-checkout-subpath.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+#
+# Copyright (C) 2011,2013 Colin Walters <walters@verbum.org>
+# Copyright (C) 2014 Red Hat, Inc.
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the
+# Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+# Boston, MA 02111-1307, USA.
+
+set -e
+
+. $(dirname $0)/libtest.sh
+
+setup_test_repository "bare"
+echo "ok setup"
+
+echo '1..2'
+
+repopath=${test_tmpdir}/ostree-srv/gnomerepo
+
+ostree --repo=repo checkout -U --subpath=/ test2 checkedout
+
+ostree --repo=repo checkout -U --subpath=/firstfile test2 checkedout2